| Author | Tokens | Token Proportion | Commits | Commit Proportion |
|---|---|---|---|---|
| Neal Cardwell | 3924 | 81.50% | 15 | 42.86% |
| Priyaranjan Jha | 608 | 12.63% | 2 | 5.71% |
| Kevin Yang | 122 | 2.53% | 4 | 11.43% |
| Eric Dumazet | 119 | 2.47% | 9 | 25.71% |
| Yousuk Seung | 30 | 0.62% | 1 | 2.86% |
| Yuchung Cheng | 7 | 0.15% | 1 | 2.86% |
| Wen Yang | 2 | 0.04% | 1 | 2.86% |
| Luke Hsiao | 2 | 0.04% | 1 | 2.86% |
| Ryan Sharpelletti | 1 | 0.02% | 1 | 2.86% |
| Total | 4815 | 35 |
/* Bottleneck Bandwidth and RTT (BBR) congestion control * * BBR congestion control computes the sending rate based on the delivery * rate (throughput) estimated from ACKs. In a nutshell: * * On each ACK, update our model of the network path: * bottleneck_bandwidth = windowed_max(delivered / elapsed, 10 round trips) * min_rtt = windowed_min(rtt, 10 seconds) * pacing_rate = pacing_gain * bottleneck_bandwidth * cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4) * * The core algorithm does not react directly to packet losses or delays, * although BBR may adjust the size of next send per ACK when loss is * observed, or adjust the sending rate if it estimates there is a * traffic policer, in order to keep the drop rate reasonable. * * Here is a state transition diagram for BBR: * * | * V * +---> STARTUP ----+ * | | | * | V | * | DRAIN ----+ * | | | * | V | * +---> PROBE_BW ----+ * | ^ | | * | | | | * | +----+ | * | | * +---- PROBE_RTT <--+ * * A BBR flow starts in STARTUP, and ramps up its sending rate quickly. * When it estimates the pipe is full, it enters DRAIN to drain the queue. * In steady state a BBR flow only uses PROBE_BW and PROBE_RTT. * A long-lived BBR flow spends the vast majority of its time remaining * (repeatedly) in PROBE_BW, fully probing and utilizing the pipe's bandwidth * in a fair manner, with a small, bounded queue. *If* a flow has been * continuously sending for the entire min_rtt window, and hasn't seen an RTT * sample that matches or decreases its min_rtt estimate for 10 seconds, then * it briefly enters PROBE_RTT to cut inflight to a minimum value to re-probe * the path's two-way propagation delay (min_rtt). When exiting PROBE_RTT, if * we estimated that we reached the full bw of the pipe then we enter PROBE_BW; * otherwise we enter STARTUP to try to fill the pipe. * * BBR is described in detail in: * "BBR: Congestion-Based Congestion Control", * Neal Cardwell, Yuchung Cheng, C. Stephen Gunn, Soheil Hassas Yeganeh, * Van Jacobson. ACM Queue, Vol. 14 No. 5, September-October 2016. * * There is a public e-mail list for discussing BBR development and testing: * https://groups.google.com/forum/#!forum/bbr-dev * * NOTE: BBR might be used with the fq qdisc ("man tc-fq") with pacing enabled, * otherwise TCP stack falls back to an internal pacing using one high * resolution timer per TCP socket and may use more resources. */ #include <linux/module.h> #include <net/tcp.h> #include <linux/inet_diag.h> #include <linux/inet.h> #include <linux/random.h> #include <linux/win_minmax.h> /* Scale factor for rate in pkt/uSec unit to avoid truncation in bandwidth * estimation. The rate unit ~= (1500 bytes / 1 usec / 2^24) ~= 715 bps. * This handles bandwidths from 0.06pps (715bps) to 256Mpps (3Tbps) in a u32. * Since the minimum window is >=4 packets, the lower bound isn't * an issue. The upper bound isn't an issue with existing technologies. */ #define BW_SCALE 24 #define BW_UNIT (1 << BW_SCALE) #define BBR_SCALE 8 /* scaling factor for fractions in BBR (e.g. gains) */ #define BBR_UNIT (1 << BBR_SCALE) /* BBR has the following modes for deciding how fast to send: */ enum bbr_mode { BBR_STARTUP, /* ramp up sending rate rapidly to fill pipe */ BBR_DRAIN, /* drain any queue created during startup */ BBR_PROBE_BW, /* discover, share bw: pace around estimated bw */ BBR_PROBE_RTT, /* cut inflight to min to probe min_rtt */ }; /* BBR congestion control block */ struct bbr { u32 min_rtt_us; /* min RTT in min_rtt_win_sec window */ u32 min_rtt_stamp; /* timestamp of min_rtt_us */ u32 probe_rtt_done_stamp; /* end time for BBR_PROBE_RTT mode */ struct minmax bw; /* Max recent delivery rate in pkts/uS << 24 */ u32 rtt_cnt; /* count of packet-timed rounds elapsed */ u32 next_rtt_delivered; /* scb->tx.delivered at end of round */ u64 cycle_mstamp; /* time of this cycle phase start */ u32 mode:3, /* current bbr_mode in state machine */ prev_ca_state:3, /* CA state on previous ACK */ packet_conservation:1, /* use packet conservation? */ round_start:1, /* start of packet-timed tx->ack round? */ idle_restart:1, /* restarting after idle? */ probe_rtt_round_done:1, /* a BBR_PROBE_RTT round at 4 pkts? */ unused:13, lt_is_sampling:1, /* taking long-term ("LT") samples now? */ lt_rtt_cnt:7, /* round trips in long-term interval */ lt_use_bw:1; /* use lt_bw as our bw estimate? */ u32 lt_bw; /* LT est delivery rate in pkts/uS << 24 */ u32 lt_last_delivered; /* LT intvl start: tp->delivered */ u32 lt_last_stamp; /* LT intvl start: tp->delivered_mstamp */ u32 lt_last_lost; /* LT intvl start: tp->lost */ u32 pacing_gain:10, /* current gain for setting pacing rate */ cwnd_gain:10, /* current gain for setting cwnd */ full_bw_reached:1, /* reached full bw in Startup? */ full_bw_cnt:2, /* number of rounds without large bw gains */ cycle_idx:3, /* current index in pacing_gain cycle array */ has_seen_rtt:1, /* have we seen an RTT sample yet? */ unused_b:5; u32 prior_cwnd; /* prior cwnd upon entering loss recovery */ u32 full_bw; /* recent bw, to estimate if pipe is full */ /* For tracking ACK aggregation: */ u64 ack_epoch_mstamp; /* start of ACK sampling epoch */ u16 extra_acked[2]; /* max excess data ACKed in epoch */ u32 ack_epoch_acked:20, /* packets (S)ACKed in sampling epoch */ extra_acked_win_rtts:5, /* age of extra_acked, in round trips */ extra_acked_win_idx:1, /* current index in extra_acked array */ unused_c:6; }; #define CYCLE_LEN 8 /* number of phases in a pacing gain cycle */ /* Window length of bw filter (in rounds): */ static const int bbr_bw_rtts = CYCLE_LEN + 2; /* Window length of min_rtt filter (in sec): */ static const u32 bbr_min_rtt_win_sec = 10; /* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */ static const u32 bbr_probe_rtt_mode_ms = 200; /* Skip TSO below the following bandwidth (bits/sec): */ static const int bbr_min_tso_rate = 1200000; /* Pace at ~1% below estimated bw, on average, to reduce queue at bottleneck. * In order to help drive the network toward lower queues and low latency while * maintaining high utilization, the average pacing rate aims to be slightly * lower than the estimated bandwidth. This is an important aspect of the * design. */ static const int bbr_pacing_margin_percent = 1; /* We use a high_gain value of 2/ln(2) because it's the smallest pacing gain * that will allow a smoothly increasing pacing rate that will double each RTT * and send the same number of packets per RTT that an un-paced, slow-starting * Reno or CUBIC flow would: */ static const int bbr_high_gain = BBR_UNIT * 2885 / 1000 + 1; /* The pacing gain of 1/high_gain in BBR_DRAIN is calculated to typically drain * the queue created in BBR_STARTUP in a single round: */ static const int bbr_drain_gain = BBR_UNIT * 1000 / 2885; /* The gain for deriving steady-state cwnd tolerates delayed/stretched ACKs: */ static const int bbr_cwnd_gain = BBR_UNIT * 2; /* The pacing_gain values for the PROBE_BW gain cycle, to discover/share bw: */ static const int bbr_pacing_gain[] = { BBR_UNIT * 5 / 4, /* probe for more available bw */ BBR_UNIT * 3 / 4, /* drain queue and/or yield bw to other flows */ BBR_UNIT, BBR_UNIT, BBR_UNIT, /* cruise at 1.0*bw to utilize pipe, */ BBR_UNIT, BBR_UNIT, BBR_UNIT /* without creating excess queue... */ }; /* Randomize the starting gain cycling phase over N phases: */ static const u32 bbr_cycle_rand = 7; /* Try to keep at least this many packets in flight, if things go smoothly. For * smooth functioning, a sliding window protocol ACKing every other packet * needs at least 4 packets in flight: */ static const u32 bbr_cwnd_min_target = 4; /* To estimate if BBR_STARTUP mode (i.e. high_gain) has filled pipe... */ /* If bw has increased significantly (1.25x), there may be more bw available: */ static const u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4; /* But after 3 rounds w/o significant bw growth, estimate pipe is full: */ static const u32 bbr_full_bw_cnt = 3; /* "long-term" ("LT") bandwidth estimator parameters... */ /* The minimum number of rounds in an LT bw sampling interval: */ static const u32 bbr_lt_intvl_min_rtts = 4; /* If lost/delivered ratio > 20%, interval is "lossy" and we may be policed: */ static const u32 bbr_lt_loss_thresh = 50; /* If 2 intervals have a bw ratio <= 1/8, their bw is "consistent": */ static const u32 bbr_lt_bw_ratio = BBR_UNIT / 8; /* If 2 intervals have a bw diff <= 4 Kbit/sec their bw is "consistent": */ static const u32 bbr_lt_bw_diff = 4000 / 8; /* If we estimate we're policed, use lt_bw for this many round trips: */ static const u32 bbr_lt_bw_max_rtts = 48; /* Gain factor for adding extra_acked to target cwnd: */ static const int bbr_extra_acked_gain = BBR_UNIT; /* Window length of extra_acked window. */ static const u32 bbr_extra_acked_win_rtts = 5; /* Max allowed val for ack_epoch_acked, after which sampling epoch is reset */ static const u32 bbr_ack_epoch_acked_reset_thresh = 1U << 20; /* Time period for clamping cwnd increment due to ack aggregation */ static const u32 bbr_extra_acked_max_us = 100 * 1000; static void bbr_check_probe_rtt_done(struct sock *sk); /* Do we estimate that STARTUP filled the pipe? */ static bool bbr_full_bw_reached(const struct sock *sk) { const struct bbr *bbr = inet_csk_ca(sk); return bbr->full_bw_reached; } /* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */ static u32 bbr_max_bw(const struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); return minmax_get(&bbr->bw); } /* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */ static u32 bbr_bw(const struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk); } /* Return maximum extra acked in past k-2k round trips, * where k = bbr_extra_acked_win_rtts. */ static u16 bbr_extra_acked(const struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); return max(bbr->extra_acked[0], bbr->extra_acked[1]); } /* Return rate in bytes per second, optionally with a gain. * The order here is chosen carefully to avoid overflow of u64. This should * work for input rates of up to 2.9Tbit/sec and gain of 2.89x. */ static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain) { unsigned int mss = tcp_sk(sk)->mss_cache; rate *= mss; rate *= gain; rate >>= BBR_SCALE; rate *= USEC_PER_SEC / 100 * (100 - bbr_pacing_margin_percent); return rate >> BW_SCALE; } /* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */ static unsigned long bbr_bw_to_pacing_rate(struct sock *sk, u32 bw, int gain) { u64 rate = bw; rate = bbr_rate_bytes_per_sec(sk, rate, gain); rate = min_t(u64, rate, sk->sk_max_pacing_rate); return rate; } /* Initialize pacing rate to: high_gain * init_cwnd / RTT. */ static void bbr_init_pacing_rate_from_rtt(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u64 bw; u32 rtt_us; if (tp->srtt_us) { /* any RTT sample yet? */ rtt_us = max(tp->srtt_us >> 3, 1U); bbr->has_seen_rtt = 1; } else { /* no RTT sample yet */ rtt_us = USEC_PER_MSEC; /* use nominal default RTT */ } bw = (u64)tp->snd_cwnd * BW_UNIT; do_div(bw, rtt_us); sk->sk_pacing_rate = bbr_bw_to_pacing_rate(sk, bw, bbr_high_gain); } /* Pace using current bw estimate and a gain factor. */ static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); unsigned long rate = bbr_bw_to_pacing_rate(sk, bw, gain); if (unlikely(!bbr->has_seen_rtt && tp->srtt_us)) bbr_init_pacing_rate_from_rtt(sk); if (bbr_full_bw_reached(sk) || rate > sk->sk_pacing_rate) sk->sk_pacing_rate = rate; } /* override sysctl_tcp_min_tso_segs */ static u32 bbr_min_tso_segs(struct sock *sk) { return sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2; } static u32 bbr_tso_segs_goal(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); u32 segs, bytes; /* Sort of tcp_tso_autosize() but ignoring * driver provided sk_gso_max_size. */ bytes = min_t(unsigned long, sk->sk_pacing_rate >> READ_ONCE(sk->sk_pacing_shift), GSO_MAX_SIZE - 1 - MAX_TCP_HEADER); segs = max_t(u32, bytes / tp->mss_cache, bbr_min_tso_segs(sk)); return min(segs, 0x7FU); } /* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */ static void bbr_save_cwnd(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT) bbr->prior_cwnd = tp->snd_cwnd; /* this cwnd is good enough */ else /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */ bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd); } static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); if (event == CA_EVENT_TX_START && tp->app_limited) { bbr->idle_restart = 1; bbr->ack_epoch_mstamp = tp->tcp_mstamp; bbr->ack_epoch_acked = 0; /* Avoid pointless buffer overflows: pace at est. bw if we don't * need more speed (we're restarting from idle and app-limited). */ if (bbr->mode == BBR_PROBE_BW) bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT); else if (bbr->mode == BBR_PROBE_RTT) bbr_check_probe_rtt_done(sk); } } /* Calculate bdp based on min RTT and the estimated bottleneck bandwidth: * * bdp = ceil(bw * min_rtt * gain) * * The key factor, gain, controls the amount of queue. While a small gain * builds a smaller queue, it becomes more vulnerable to noise in RTT * measurements (e.g., delayed ACKs or other ACK compression effects). This * noise may cause BBR to under-estimate the rate. */ static u32 bbr_bdp(struct sock *sk, u32 bw, int gain) { struct bbr *bbr = inet_csk_ca(sk); u32 bdp; u64 w; /* If we've never had a valid RTT sample, cap cwnd at the initial * default. This should only happen when the connection is not using TCP * timestamps and has retransmitted all of the SYN/SYNACK/data packets * ACKed so far. In this case, an RTO can cut cwnd to 1, in which * case we need to slow-start up toward something safe: TCP_INIT_CWND. */ if (unlikely(bbr->min_rtt_us == ~0U)) /* no valid RTT samples yet? */ return TCP_INIT_CWND; /* be safe: cap at default initial cwnd*/ w = (u64)bw * bbr->min_rtt_us; /* Apply a gain to the given value, remove the BW_SCALE shift, and * round the value up to avoid a negative feedback loop. */ bdp = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT; return bdp; } /* To achieve full performance in high-speed paths, we budget enough cwnd to * fit full-sized skbs in-flight on both end hosts to fully utilize the path: * - one skb in sending host Qdisc, * - one skb in sending host TSO/GSO engine * - one skb being received by receiver host LRO/GRO/delayed-ACK engine * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets, * which allows 2 outstanding 2-packet sequences, to try to keep pipe * full even with ACK-every-other-packet delayed ACKs. */ static u32 bbr_quantization_budget(struct sock *sk, u32 cwnd) { struct bbr *bbr = inet_csk_ca(sk); /* Allow enough full-sized skbs in flight to utilize end systems. */ cwnd += 3 * bbr_tso_segs_goal(sk); /* Reduce delayed ACKs by rounding up cwnd to the next even number. */ cwnd = (cwnd + 1) & ~1U; /* Ensure gain cycling gets inflight above BDP even for small BDPs. */ if (bbr->mode == BBR_PROBE_BW && bbr->cycle_idx == 0) cwnd += 2; return cwnd; } /* Find inflight based on min RTT and the estimated bottleneck bandwidth. */ static u32 bbr_inflight(struct sock *sk, u32 bw, int gain) { u32 inflight; inflight = bbr_bdp(sk, bw, gain); inflight = bbr_quantization_budget(sk, inflight); return inflight; } /* With pacing at lower layers, there's often less data "in the network" than * "in flight". With TSQ and departure time pacing at lower layers (e.g. fq), * we often have several skbs queued in the pacing layer with a pre-scheduled * earliest departure time (EDT). BBR adapts its pacing rate based on the * inflight level that it estimates has already been "baked in" by previous * departure time decisions. We calculate a rough estimate of the number of our * packets that might be in the network at the earliest departure time for the * next skb scheduled: * in_network_at_edt = inflight_at_edt - (EDT - now) * bw * If we're increasing inflight, then we want to know if the transmit of the * EDT skb will push inflight above the target, so inflight_at_edt includes * bbr_tso_segs_goal() from the skb departing at EDT. If decreasing inflight, * then estimate if inflight will sink too low just before the EDT transmit. */ static u32 bbr_packets_in_net_at_edt(struct sock *sk, u32 inflight_now) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u64 now_ns, edt_ns, interval_us; u32 interval_delivered, inflight_at_edt; now_ns = tp->tcp_clock_cache; edt_ns = max(tp->tcp_wstamp_ns, now_ns); interval_us = div_u64(edt_ns - now_ns, NSEC_PER_USEC); interval_delivered = (u64)bbr_bw(sk) * interval_us >> BW_SCALE; inflight_at_edt = inflight_now; if (bbr->pacing_gain > BBR_UNIT) /* increasing inflight */ inflight_at_edt += bbr_tso_segs_goal(sk); /* include EDT skb */ if (interval_delivered >= inflight_at_edt) return 0; return inflight_at_edt - interval_delivered; } /* Find the cwnd increment based on estimate of ack aggregation */ static u32 bbr_ack_aggregation_cwnd(struct sock *sk) { u32 max_aggr_cwnd, aggr_cwnd = 0; if (bbr_extra_acked_gain && bbr_full_bw_reached(sk)) { max_aggr_cwnd = ((u64)bbr_bw(sk) * bbr_extra_acked_max_us) / BW_UNIT; aggr_cwnd = (bbr_extra_acked_gain * bbr_extra_acked(sk)) >> BBR_SCALE; aggr_cwnd = min(aggr_cwnd, max_aggr_cwnd); } return aggr_cwnd; } /* An optimization in BBR to reduce losses: On the first round of recovery, we * follow the packet conservation principle: send P packets per P packets acked. * After that, we slow-start and send at most 2*P packets per P packets acked. * After recovery finishes, or upon undo, we restore the cwnd we had when * recovery started (capped by the target cwnd based on estimated BDP). * * TODO(ycheng/ncardwell): implement a rate-based approach. */ static bool bbr_set_cwnd_to_recover_or_restore( struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state; u32 cwnd = tp->snd_cwnd; /* An ACK for P pkts should release at most 2*P packets. We do this * in two steps. First, here we deduct the number of lost packets. * Then, in bbr_set_cwnd() we slow start up toward the target cwnd. */ if (rs->losses > 0) cwnd = max_t(s32, cwnd - rs->losses, 1); if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) { /* Starting 1st round of Recovery, so do packet conservation. */ bbr->packet_conservation = 1; bbr->next_rtt_delivered = tp->delivered; /* start round now */ /* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */ cwnd = tcp_packets_in_flight(tp) + acked; } else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) { /* Exiting loss recovery; restore cwnd saved before recovery. */ cwnd = max(cwnd, bbr->prior_cwnd); bbr->packet_conservation = 0; } bbr->prev_ca_state = state; if (bbr->packet_conservation) { *new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked); return true; /* yes, using packet conservation */ } *new_cwnd = cwnd; return false; } /* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss * has drawn us down below target), or snap down to target if we're above it. */ static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs, u32 acked, u32 bw, int gain) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u32 cwnd = tp->snd_cwnd, target_cwnd = 0; if (!acked) goto done; /* no packet fully ACKed; just apply caps */ if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd)) goto done; target_cwnd = bbr_bdp(sk, bw, gain); /* Increment the cwnd to account for excess ACKed data that seems * due to aggregation (of data and/or ACKs) visible in the ACK stream. */ target_cwnd += bbr_ack_aggregation_cwnd(sk); target_cwnd = bbr_quantization_budget(sk, target_cwnd); /* If we're below target cwnd, slow start cwnd toward target cwnd. */ if (bbr_full_bw_reached(sk)) /* only cut cwnd if we filled the pipe */ cwnd = min(cwnd + acked, target_cwnd); else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND) cwnd = cwnd + acked; cwnd = max(cwnd, bbr_cwnd_min_target); done: tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp); /* apply global cap */ if (bbr->mode == BBR_PROBE_RTT) /* drain queue, refresh min_rtt */ tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target); } /* End cycle phase if it's time and/or we hit the phase's in-flight target. */ static bool bbr_is_next_cycle_phase(struct sock *sk, const struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bool is_full_length = tcp_stamp_us_delta(tp->delivered_mstamp, bbr->cycle_mstamp) > bbr->min_rtt_us; u32 inflight, bw; /* The pacing_gain of 1.0 paces at the estimated bw to try to fully * use the pipe without increasing the queue. */ if (bbr->pacing_gain == BBR_UNIT) return is_full_length; /* just use wall clock time */ inflight = bbr_packets_in_net_at_edt(sk, rs->prior_in_flight); bw = bbr_max_bw(sk); /* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is * small (e.g. on a LAN). We do not persist if packets are lost, since * a path with small buffers may not hold that much. */ if (bbr->pacing_gain > BBR_UNIT) return is_full_length && (rs->losses || /* perhaps pacing_gain*BDP won't fit */ inflight >= bbr_inflight(sk, bw, bbr->pacing_gain)); /* A pacing_gain < 1.0 tries to drain extra queue we added if bw * probing didn't find more bw. If inflight falls to match BDP then we * estimate queue is drained; persisting would underutilize the pipe. */ return is_full_length || inflight <= bbr_inflight(sk, bw, BBR_UNIT); } static void bbr_advance_cycle_phase(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1); bbr->cycle_mstamp = tp->delivered_mstamp; } /* Gain cycling: cycle pacing gain to converge to fair share of available bw. */ static void bbr_update_cycle_phase(struct sock *sk, const struct rate_sample *rs) { struct bbr *bbr = inet_csk_ca(sk); if (bbr->mode == BBR_PROBE_BW && bbr_is_next_cycle_phase(sk, rs)) bbr_advance_cycle_phase(sk); } static void bbr_reset_startup_mode(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->mode = BBR_STARTUP; } static void bbr_reset_probe_bw_mode(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->mode = BBR_PROBE_BW; bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand); bbr_advance_cycle_phase(sk); /* flip to next phase of gain cycle */ } static void bbr_reset_mode(struct sock *sk) { if (!bbr_full_bw_reached(sk)) bbr_reset_startup_mode(sk); else bbr_reset_probe_bw_mode(sk); } /* Start a new long-term sampling interval. */ static void bbr_reset_lt_bw_sampling_interval(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bbr->lt_last_stamp = div_u64(tp->delivered_mstamp, USEC_PER_MSEC); bbr->lt_last_delivered = tp->delivered; bbr->lt_last_lost = tp->lost; bbr->lt_rtt_cnt = 0; } /* Completely reset long-term bandwidth sampling. */ static void bbr_reset_lt_bw_sampling(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->lt_bw = 0; bbr->lt_use_bw = 0; bbr->lt_is_sampling = false; bbr_reset_lt_bw_sampling_interval(sk); } /* Long-term bw sampling interval is done. Estimate whether we're policed. */ static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw) { struct bbr *bbr = inet_csk_ca(sk); u32 diff; if (bbr->lt_bw) { /* do we have bw from a previous interval? */ /* Is new bw close to the lt_bw from the previous interval? */ diff = abs(bw - bbr->lt_bw); if ((diff * BBR_UNIT <= bbr_lt_bw_ratio * bbr->lt_bw) || (bbr_rate_bytes_per_sec(sk, diff, BBR_UNIT) <= bbr_lt_bw_diff)) { /* All criteria are met; estimate we're policed. */ bbr->lt_bw = (bw + bbr->lt_bw) >> 1; /* avg 2 intvls */ bbr->lt_use_bw = 1; bbr->pacing_gain = BBR_UNIT; /* try to avoid drops */ bbr->lt_rtt_cnt = 0; return; } } bbr->lt_bw = bw; bbr_reset_lt_bw_sampling_interval(sk); } /* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and * explicitly models their policed rate, to reduce unnecessary losses. We * estimate that we're policed if we see 2 consecutive sampling intervals with * consistent throughput and high packet loss. If we think we're being policed, * set lt_bw to the "long-term" average delivery rate from those 2 intervals. */ static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u32 lost, delivered; u64 bw; u32 t; if (bbr->lt_use_bw) { /* already using long-term rate, lt_bw? */ if (bbr->mode == BBR_PROBE_BW && bbr->round_start && ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) { bbr_reset_lt_bw_sampling(sk); /* stop using lt_bw */ bbr_reset_probe_bw_mode(sk); /* restart gain cycling */ } return; } /* Wait for the first loss before sampling, to let the policer exhaust * its tokens and estimate the steady-state rate allowed by the policer. * Starting samples earlier includes bursts that over-estimate the bw. */ if (!bbr->lt_is_sampling) { if (!rs->losses) return; bbr_reset_lt_bw_sampling_interval(sk); bbr->lt_is_sampling = true; } /* To avoid underestimates, reset sampling if we run out of data. */ if (rs->is_app_limited) { bbr_reset_lt_bw_sampling(sk); return; } if (bbr->round_start) bbr->lt_rtt_cnt++; /* count round trips in this interval */ if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts) return; /* sampling interval needs to be longer */ if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) { bbr_reset_lt_bw_sampling(sk); /* interval is too long */ return; } /* End sampling interval when a packet is lost, so we estimate the * policer tokens were exhausted. Stopping the sampling before the * tokens are exhausted under-estimates the policed rate. */ if (!rs->losses) return; /* Calculate packets lost and delivered in sampling interval. */ lost = tp->lost - bbr->lt_last_lost; delivered = tp->delivered - bbr->lt_last_delivered; /* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */ if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered) return; /* Find average delivery rate in this sampling interval. */ t = div_u64(tp->delivered_mstamp, USEC_PER_MSEC) - bbr->lt_last_stamp; if ((s32)t < 1) return; /* interval is less than one ms, so wait */ /* Check if can multiply without overflow */ if (t >= ~0U / USEC_PER_MSEC) { bbr_reset_lt_bw_sampling(sk); /* interval too long; reset */ return; } t *= USEC_PER_MSEC; bw = (u64)delivered * BW_UNIT; do_div(bw, t); bbr_lt_bw_interval_done(sk, bw); } /* Estimate the bandwidth based on how fast packets are delivered */ static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u64 bw; bbr->round_start = 0; if (rs->delivered < 0 || rs->interval_us <= 0) return; /* Not a valid observation */ /* See if we've reached the next RTT */ if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) { bbr->next_rtt_delivered = tp->delivered; bbr->rtt_cnt++; bbr->round_start = 1; bbr->packet_conservation = 0; } bbr_lt_bw_sampling(sk, rs); /* Divide delivered by the interval to find a (lower bound) bottleneck * bandwidth sample. Delivered is in packets and interval_us in uS and * ratio will be <<1 for most connections. So delivered is first scaled. */ bw = div64_long((u64)rs->delivered * BW_UNIT, rs->interval_us); /* If this sample is application-limited, it is likely to have a very * low delivered count that represents application behavior rather than * the available network rate. Such a sample could drag down estimated * bw, causing needless slow-down. Thus, to continue to send at the * last measured network rate, we filter out app-limited samples unless * they describe the path bw at least as well as our bw model. * * So the goal during app-limited phase is to proceed with the best * network rate no matter how long. We automatically leave this * phase when app writes faster than the network can deliver :) */ if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) { /* Incorporate new sample into our max bw filter. */ minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw); } } /* Estimates the windowed max degree of ack aggregation. * This is used to provision extra in-flight data to keep sending during * inter-ACK silences. * * Degree of ack aggregation is estimated as extra data acked beyond expected. * * max_extra_acked = "maximum recent excess data ACKed beyond max_bw * interval" * cwnd += max_extra_acked * * Max extra_acked is clamped by cwnd and bw * bbr_extra_acked_max_us (100 ms). * Max filter is an approximate sliding window of 5-10 (packet timed) round * trips. */ static void bbr_update_ack_aggregation(struct sock *sk, const struct rate_sample *rs) { u32 epoch_us, expected_acked, extra_acked; struct bbr *bbr = inet_csk_ca(sk); struct tcp_sock *tp = tcp_sk(sk); if (!bbr_extra_acked_gain || rs->acked_sacked <= 0 || rs->delivered < 0 || rs->interval_us <= 0) return; if (bbr->round_start) { bbr->extra_acked_win_rtts = min(0x1F, bbr->extra_acked_win_rtts + 1); if (bbr->extra_acked_win_rtts >= bbr_extra_acked_win_rtts) { bbr->extra_acked_win_rtts = 0; bbr->extra_acked_win_idx = bbr->extra_acked_win_idx ? 0 : 1; bbr->extra_acked[bbr->extra_acked_win_idx] = 0; } } /* Compute how many packets we expected to be delivered over epoch. */ epoch_us = tcp_stamp_us_delta(tp->delivered_mstamp, bbr->ack_epoch_mstamp); expected_acked = ((u64)bbr_bw(sk) * epoch_us) / BW_UNIT; /* Reset the aggregation epoch if ACK rate is below expected rate or * significantly large no. of ack received since epoch (potentially * quite old epoch). */ if (bbr->ack_epoch_acked <= expected_acked || (bbr->ack_epoch_acked + rs->acked_sacked >= bbr_ack_epoch_acked_reset_thresh)) { bbr->ack_epoch_acked = 0; bbr->ack_epoch_mstamp = tp->delivered_mstamp; expected_acked = 0; } /* Compute excess data delivered, beyond what was expected. */ bbr->ack_epoch_acked = min_t(u32, 0xFFFFF, bbr->ack_epoch_acked + rs->acked_sacked); extra_acked = bbr->ack_epoch_acked - expected_acked; extra_acked = min(extra_acked, tp->snd_cwnd); if (extra_acked > bbr->extra_acked[bbr->extra_acked_win_idx]) bbr->extra_acked[bbr->extra_acked_win_idx] = extra_acked; } /* Estimate when the pipe is full, using the change in delivery rate: BBR * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the * higher rwin, 3: we get higher delivery rate samples. Or transient * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar * design goal, but uses delay and inter-ACK spacing instead of bandwidth. */ static void bbr_check_full_bw_reached(struct sock *sk, const struct rate_sample *rs) { struct bbr *bbr = inet_csk_ca(sk); u32 bw_thresh; if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited) return; bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE; if (bbr_max_bw(sk) >= bw_thresh) { bbr->full_bw = bbr_max_bw(sk); bbr->full_bw_cnt = 0; return; } ++bbr->full_bw_cnt; bbr->full_bw_reached = bbr->full_bw_cnt >= bbr_full_bw_cnt; } /* If pipe is probably full, drain the queue and then enter steady-state. */ static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs) { struct bbr *bbr = inet_csk_ca(sk); if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) { bbr->mode = BBR_DRAIN; /* drain queue we created */ tcp_sk(sk)->snd_ssthresh = bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT); } /* fall through to check if in-flight is already small: */ if (bbr->mode == BBR_DRAIN && bbr_packets_in_net_at_edt(sk, tcp_packets_in_flight(tcp_sk(sk))) <= bbr_inflight(sk, bbr_max_bw(sk), BBR_UNIT)) bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */ } static void bbr_check_probe_rtt_done(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); if (!(bbr->probe_rtt_done_stamp && after(tcp_jiffies32, bbr->probe_rtt_done_stamp))) return; bbr->min_rtt_stamp = tcp_jiffies32; /* wait a while until PROBE_RTT */ tp->snd_cwnd = max(tp->snd_cwnd, bbr->prior_cwnd); bbr_reset_mode(sk); } /* The goal of PROBE_RTT mode is to have BBR flows cooperatively and * periodically drain the bottleneck queue, to converge to measure the true * min_rtt (unloaded propagation delay). This allows the flows to keep queues * small (reducing queuing delay and packet loss) and achieve fairness among * BBR flows. * * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires, * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets. * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and * re-enter the previous mode. BBR uses 200ms to approximately bound the * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s). * * Note that flows need only pay 2% if they are busy sending over the last 10 * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have * natural silences or low-rate periods within 10 seconds where the rate is low * enough for long enough to drain its queue in the bottleneck. We pick up * these min RTT measurements opportunistically with our min_rtt filter. :-) */ static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bool filter_expired; /* Track min RTT seen in the min_rtt_win_sec filter window: */ filter_expired = after(tcp_jiffies32, bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ); if (rs->rtt_us >= 0 && (rs->rtt_us < bbr->min_rtt_us || (filter_expired && !rs->is_ack_delayed))) { bbr->min_rtt_us = rs->rtt_us; bbr->min_rtt_stamp = tcp_jiffies32; } if (bbr_probe_rtt_mode_ms > 0 && filter_expired && !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) { bbr->mode = BBR_PROBE_RTT; /* dip, drain queue */ bbr_save_cwnd(sk); /* note cwnd so we can restore it */ bbr->probe_rtt_done_stamp = 0; } if (bbr->mode == BBR_PROBE_RTT) { /* Ignore low rate samples during this mode. */ tp->app_limited = (tp->delivered + tcp_packets_in_flight(tp)) ? : 1; /* Maintain min packets in flight for max(200 ms, 1 round). */ if (!bbr->probe_rtt_done_stamp && tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) { bbr->probe_rtt_done_stamp = tcp_jiffies32 + msecs_to_jiffies(bbr_probe_rtt_mode_ms); bbr->probe_rtt_round_done = 0; bbr->next_rtt_delivered = tp->delivered; } else if (bbr->probe_rtt_done_stamp) { if (bbr->round_start) bbr->probe_rtt_round_done = 1; if (bbr->probe_rtt_round_done) bbr_check_probe_rtt_done(sk); } } /* Restart after idle ends only once we process a new S/ACK for data */ if (rs->delivered > 0) bbr->idle_restart = 0; } static void bbr_update_gains(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); switch (bbr->mode) { case BBR_STARTUP: bbr->pacing_gain = bbr_high_gain; bbr->cwnd_gain = bbr_high_gain; break; case BBR_DRAIN: bbr->pacing_gain = bbr_drain_gain; /* slow, to drain */ bbr->cwnd_gain = bbr_high_gain; /* keep cwnd */ break; case BBR_PROBE_BW: bbr->pacing_gain = (bbr->lt_use_bw ? BBR_UNIT : bbr_pacing_gain[bbr->cycle_idx]); bbr->cwnd_gain = bbr_cwnd_gain; break; case BBR_PROBE_RTT: bbr->pacing_gain = BBR_UNIT; bbr->cwnd_gain = BBR_UNIT; break; default: WARN_ONCE(1, "BBR bad mode: %u\n", bbr->mode); break; } } static void bbr_update_model(struct sock *sk, const struct rate_sample *rs) { bbr_update_bw(sk, rs); bbr_update_ack_aggregation(sk, rs); bbr_update_cycle_phase(sk, rs); bbr_check_full_bw_reached(sk, rs); bbr_check_drain(sk, rs); bbr_update_min_rtt(sk, rs); bbr_update_gains(sk); } static void bbr_main(struct sock *sk, const struct rate_sample *rs) { struct bbr *bbr = inet_csk_ca(sk); u32 bw; bbr_update_model(sk, rs); bw = bbr_bw(sk); bbr_set_pacing_rate(sk, bw, bbr->pacing_gain); bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain); } static void bbr_init(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); bbr->prior_cwnd = 0; tp->snd_ssthresh = TCP_INFINITE_SSTHRESH; bbr->rtt_cnt = 0; bbr->next_rtt_delivered = 0; bbr->prev_ca_state = TCP_CA_Open; bbr->packet_conservation = 0; bbr->probe_rtt_done_stamp = 0; bbr->probe_rtt_round_done = 0; bbr->min_rtt_us = tcp_min_rtt(tp); bbr->min_rtt_stamp = tcp_jiffies32; minmax_reset(&bbr->bw, bbr->rtt_cnt, 0); /* init max bw to 0 */ bbr->has_seen_rtt = 0; bbr_init_pacing_rate_from_rtt(sk); bbr->round_start = 0; bbr->idle_restart = 0; bbr->full_bw_reached = 0; bbr->full_bw = 0; bbr->full_bw_cnt = 0; bbr->cycle_mstamp = 0; bbr->cycle_idx = 0; bbr_reset_lt_bw_sampling(sk); bbr_reset_startup_mode(sk); bbr->ack_epoch_mstamp = tp->tcp_mstamp; bbr->ack_epoch_acked = 0; bbr->extra_acked_win_rtts = 0; bbr->extra_acked_win_idx = 0; bbr->extra_acked[0] = 0; bbr->extra_acked[1] = 0; cmpxchg(&sk->sk_pacing_status, SK_PACING_NONE, SK_PACING_NEEDED); } static u32 bbr_sndbuf_expand(struct sock *sk) { /* Provision 3 * cwnd since BBR may slow-start even during recovery. */ return 3; } /* In theory BBR does not need to undo the cwnd since it does not * always reduce cwnd on losses (see bbr_main()). Keep it for now. */ static u32 bbr_undo_cwnd(struct sock *sk) { struct bbr *bbr = inet_csk_ca(sk); bbr->full_bw = 0; /* spurious slow-down; reset full pipe detection */ bbr->full_bw_cnt = 0; bbr_reset_lt_bw_sampling(sk); return tcp_sk(sk)->snd_cwnd; } /* Entering loss recovery, so save cwnd for when we exit or undo recovery. */ static u32 bbr_ssthresh(struct sock *sk) { bbr_save_cwnd(sk); return tcp_sk(sk)->snd_ssthresh; } static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr, union tcp_cc_info *info) { if (ext & (1 << (INET_DIAG_BBRINFO - 1)) || ext & (1 << (INET_DIAG_VEGASINFO - 1))) { struct tcp_sock *tp = tcp_sk(sk); struct bbr *bbr = inet_csk_ca(sk); u64 bw = bbr_bw(sk); bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE; memset(&info->bbr, 0, sizeof(info->bbr)); info->bbr.bbr_bw_lo = (u32)bw; info->bbr.bbr_bw_hi = (u32)(bw >> 32); info->bbr.bbr_min_rtt = bbr->min_rtt_us; info->bbr.bbr_pacing_gain = bbr->pacing_gain; info->bbr.bbr_cwnd_gain = bbr->cwnd_gain; *attr = INET_DIAG_BBRINFO; return sizeof(info->bbr); } return 0; } static void bbr_set_state(struct sock *sk, u8 new_state) { struct bbr *bbr = inet_csk_ca(sk); if (new_state == TCP_CA_Loss) { struct rate_sample rs = { .losses = 1 }; bbr->prev_ca_state = TCP_CA_Loss; bbr->full_bw = 0; bbr->round_start = 1; /* treat RTO like end of a round */ bbr_lt_bw_sampling(sk, &rs); } } static struct tcp_congestion_ops tcp_bbr_cong_ops __read_mostly = { .flags = TCP_CONG_NON_RESTRICTED, .name = "bbr", .owner = THIS_MODULE, .init = bbr_init, .cong_control = bbr_main, .sndbuf_expand = bbr_sndbuf_expand, .undo_cwnd = bbr_undo_cwnd, .cwnd_event = bbr_cwnd_event, .ssthresh = bbr_ssthresh, .min_tso_segs = bbr_min_tso_segs, .get_info = bbr_get_info, .set_state = bbr_set_state, }; static int __init bbr_register(void) { BUILD_BUG_ON(sizeof(struct bbr) > ICSK_CA_PRIV_SIZE); return tcp_register_congestion_control(&tcp_bbr_cong_ops); } static void __exit bbr_unregister(void) { tcp_unregister_congestion_control(&tcp_bbr_cong_ops); } module_init(bbr_register); module_exit(bbr_unregister); MODULE_AUTHOR("Van Jacobson <vanj@google.com>"); MODULE_AUTHOR("Neal Cardwell <ncardwell@google.com>"); MODULE_AUTHOR("Yuchung Cheng <ycheng@google.com>"); MODULE_AUTHOR("Soheil Hassas Yeganeh <soheil@google.com>"); MODULE_LICENSE("Dual BSD/GPL"); MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)");
Information contained on this website is for historical information purposes only and does not indicate or represent copyright ownership.
Created with Cregit http://github.com/cregit/cregit
Version 2.0-RC1