--- /dev/null
+/* 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.
+ *
+ * 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 *must* be used with the fq qdisc ("man tc-fq") with pacing enabled,
+ * since pacing is integral to the BBR design and implementation.
+ * BBR without pacing would not function properly, and may incur unnecessary
+ * high packet loss rates.
+ */
+#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 cwnd 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 */
+ struct skb_mstamp 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? */
+ restore_cwnd:1, /* decided to revert cwnd to old value */
+ round_start:1, /* start of packet-timed tx->ack round? */
+ tso_segs_goal:7, /* segments we want in each skb we send */
+ idle_restart:1, /* restarting after idle? */
+ probe_rtt_round_done:1, /* a BBR_PROBE_RTT round at 4 pkts? */
+ unused:5,
+ 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_cnt:3, /* number of rounds without large bw gains */
+ cycle_idx:3, /* current index in pacing_gain cycle array */
+ unused_b:6;
+ u32 prior_cwnd; /* prior cwnd upon entering loss recovery */
+ u32 full_bw; /* recent bw, to estimate if pipe is full */
+};
+
+#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;
+
+/* 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;
+
+/* 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_cnt >= bbr_full_bw_cnt;
+}
+
+/* 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 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)
+{
+ rate *= tcp_mss_to_mtu(sk, tcp_sk(sk)->mss_cache);
+ rate *= gain;
+ rate >>= BBR_SCALE;
+ rate *= USEC_PER_SEC;
+ return rate >> BW_SCALE;
+}
+
+/* Pace using current bw estimate and a gain factor. In order to help drive the
+ * network toward lower queues while maintaining high utilization and low
+ * latency, the average pacing rate aims to be slightly (~1%) lower than the
+ * estimated bandwidth. This is an important aspect of the design. In this
+ * implementation this slightly lower pacing rate is achieved implicitly by not
+ * including link-layer headers in the packet size used for the pacing rate.
+ */
+static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
+{
+ struct bbr *bbr = inet_csk_ca(sk);
+ u64 rate = bw;
+
+ rate = bbr_rate_bytes_per_sec(sk, rate, gain);
+ rate = min_t(u64, rate, sk->sk_max_pacing_rate);
+ if (bbr->mode != BBR_STARTUP || rate > sk->sk_pacing_rate)
+ sk->sk_pacing_rate = rate;
+}
+
+/* Return count of segments we want in the skbs we send, or 0 for default. */
+static u32 bbr_tso_segs_goal(struct sock *sk)
+{
+ struct bbr *bbr = inet_csk_ca(sk);
+
+ return bbr->tso_segs_goal;
+}
+
+static void bbr_set_tso_segs_goal(struct sock *sk)
+{
+ struct tcp_sock *tp = tcp_sk(sk);
+ struct bbr *bbr = inet_csk_ca(sk);
+ u32 min_segs;
+
+ min_segs = sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2;
+ bbr->tso_segs_goal = min(tcp_tso_autosize(sk, tp->mss_cache, 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;
+ /* 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);
+ }
+}
+
+/* Find target cwnd. Right-size the cwnd based on min RTT and the
+ * estimated bottleneck bandwidth:
+ *
+ * cwnd = bw * min_rtt * gain = BDP * 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.
+ *
+ * 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_target_cwnd(struct sock *sk, u32 bw, int gain)
+{
+ struct bbr *bbr = inet_csk_ca(sk);
+ u32 cwnd;
+ 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, then remove the BW_SCALE shift. */
+ cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
+
+ /* Allow enough full-sized skbs in flight to utilize end systems. */
+ cwnd += 3 * bbr->tso_segs_goal;
+
+ /* Reduce delayed ACKs by rounding up cwnd to the next even number. */
+ cwnd = (cwnd + 1) & ~1U;
+
+ return 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. */
+ bbr->restore_cwnd = 1;
+ bbr->packet_conservation = 0;
+ }
+ bbr->prev_ca_state = state;
+
+ if (bbr->restore_cwnd) {
+ /* Restore cwnd after exiting loss recovery or PROBE_RTT. */
+ cwnd = max(cwnd, bbr->prior_cwnd);
+ bbr->restore_cwnd = 0;
+ }
+
+ 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 = 0, target_cwnd = 0;
+
+ if (!acked)
+ return;
+
+ if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
+ goto done;
+
+ /* If we're below target cwnd, slow start cwnd toward target cwnd. */
+ target_cwnd = bbr_target_cwnd(sk, bw, gain);
+ 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 =
+ skb_mstamp_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 = rs->prior_in_flight; /* what was in-flight before ACK? */
+ 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_target_cwnd(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_target_cwnd(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;
+ bbr->pacing_gain = bbr_pacing_gain[bbr->cycle_idx];
+}
+
+/* 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->lt_use_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;
+ bbr->pacing_gain = bbr_high_gain;
+ bbr->cwnd_gain = bbr_high_gain;
+}
+
+static void bbr_reset_probe_bw_mode(struct sock *sk)
+{
+ struct bbr *bbr = inet_csk_ca(sk);
+
+ bbr->mode = BBR_PROBE_BW;
+ bbr->pacing_gain = BBR_UNIT;
+ bbr->cwnd_gain = bbr_cwnd_gain;
+ 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 = tp->delivered_mstamp.stamp_jiffies;
+ 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;
+ s32 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 = (s32)(tp->delivered_mstamp.stamp_jiffies - bbr->lt_last_stamp);
+ if (t < 1)
+ return; /* interval is less than one jiffy, so wait */
+ t = jiffies_to_usecs(t);
+ /* Interval long enough for jiffies_to_usecs() to return a bogus 0? */
+ if (t < 1) {
+ bbr_reset_lt_bw_sampling(sk); /* interval too long; reset */
+ return;
+ }
+ 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 = (u64)rs->delivered * BW_UNIT;
+ do_div(bw, 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);
+ }
+}
+
+/* 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;
+}
+
+/* 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 */
+ bbr->pacing_gain = bbr_drain_gain; /* pace slow to drain */
+ bbr->cwnd_gain = bbr_high_gain; /* maintain cwnd */
+ } /* fall through to check if in-flight is already small: */
+ if (bbr->mode == BBR_DRAIN &&
+ tcp_packets_in_flight(tcp_sk(sk)) <=
+ bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT))
+ bbr_reset_probe_bw_mode(sk); /* we estimate queue is drained */
+}
+
+/* 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_time_stamp,
+ bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
+ if (rs->rtt_us >= 0 &&
+ (rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
+ bbr->min_rtt_us = rs->rtt_us;
+ bbr->min_rtt_stamp = tcp_time_stamp;
+ }
+
+ 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->pacing_gain = BBR_UNIT;
+ bbr->cwnd_gain = BBR_UNIT;
+ 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_time_stamp +
+ 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 &&
+ after(tcp_time_stamp, bbr->probe_rtt_done_stamp)) {
+ bbr->min_rtt_stamp = tcp_time_stamp;
+ bbr->restore_cwnd = 1; /* snap to prior_cwnd */
+ bbr_reset_mode(sk);
+ }
+ }
+ }
+ bbr->idle_restart = 0;
+}
+
+static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
+{
+ bbr_update_bw(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);
+}
+
+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_tso_segs_goal(sk);
+ 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);
+ u64 bw;
+
+ bbr->prior_cwnd = 0;
+ bbr->tso_segs_goal = 0; /* default segs per skb until first ACK */
+ 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_time_stamp;
+
+ minmax_reset(&bbr->bw, bbr->rtt_cnt, 0); /* init max bw to 0 */
+
+ /* Initialize pacing rate to: high_gain * init_cwnd / RTT. */
+ bw = (u64)tp->snd_cwnd * BW_UNIT;
+ do_div(bw, (tp->srtt_us >> 3) ? : USEC_PER_MSEC);
+ sk->sk_pacing_rate = 0; /* force an update of sk_pacing_rate */
+ bbr_set_pacing_rate(sk, bw, bbr_high_gain);
+
+ bbr->restore_cwnd = 0;
+ bbr->round_start = 0;
+ bbr->idle_restart = 0;
+ bbr->full_bw = 0;
+ bbr->full_bw_cnt = 0;
+ bbr->cycle_mstamp.v64 = 0;
+ bbr->cycle_idx = 0;
+ bbr_reset_lt_bw_sampling(sk);
+ bbr_reset_startup_mode(sk);
+}
+
+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)
+{
+ 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_INFINITE_SSTHRESH; /* BBR does not use 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,
+ .tso_segs_goal = bbr_tso_segs_goal,
+ .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)");
projects / GitHub / moto-9609 / android_kernel_motorola_exynos9610.git / commitdiff