/* 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
  ", 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)");