tcp_bbr: add BBR congestion control

[linux-2.6-microblaze.git] / net / ipv4 / tcp_bbr.c

/* 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)");