Google bbr源码分析

 

状态机：

bbr算法主要流程：

代码：

   /* 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: */
  /*
      BBR 四种模式：
      STARTUP：快速开始抢占带宽
      DRAIN：清空启动期间创建的任何队列
      PROBE_BW: 周期测试最佳带宽
      PROBE_RTT:清空发出的包计算minRTT
  */
  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;         /* 周期阶段的开始 */
     u32     mode:3,             /* 当前的bbr mode */
         prev_ca_state:3,     /* CA state on previous ACK */
         packet_conservation:1,  /* 使用包保护吗？ */
         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,    /* cnwd当前增益 */    
         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;    /* 之前的cwnd */
     u32    full_bw;    /* 最近bw，估计管道是否满 */
 
     /* 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): */
 //10s未更新最小rtt则进入PROBE_RTT
 static const u32 bbr_min_rtt_win_sec = 10;
 /* Minimum time (in ms) spent at bbr_cwnd_min_target in BBR_PROBE_RTT mode: */
 //BR_PROBE_RTT 在bbr_cwnd_min_target上花费的最小时间
 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. */
 //TODO 返回窗口的最大最近带宽样本
 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. */
 //TODO 返回预估的带宽的路径 bbr->lt_use_bw == BBR_DRAIN ? bbr->lt_bw : bbr_max_bw(sk)
 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 */
 //TODO 覆盖 sysctl_tcp_min_tso_segs sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2
 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.
  */
 //return bdp = ((((u64)bw * bbr->min_rtt_us * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT
 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. */
 //TODO 增益循环 循环速度增益收敛到可用bw的公平份额
 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;
 }
 
 //TODO 进入PROBE_BW状态
 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 */
 //TODO 根据发送数据包的速度估计带宽
 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;
     
     //无效数据
     if (rs->delivered < 0 || rs->interval_us <= 0)
         return; /* Not a valid observation */
 
     /* See if we've reached the next RTT */
     // 判断是否到达下一个rtt
     if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
         //更新rtt
         bbr->next_rtt_delivered = tp->delivered;
         bbr->rtt_cnt++;
         bbr->round_start = 1;
         bbr->packet_conservation = 0;
     }
 
     //令牌桶监管
     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.
      */
     //rs->delivered: 采样期间已确认的字节数
     //rs->interval_us: 采样时间
     //带宽 = 确认的字节数 * BW_UNIT / 采样时间
     bw = div64_long((u64)rs->delivered * BW_UNIT, rs->interval_us);
     
     /*
     * static inline s64 div64_s64(s64 dividend, s64 divisor)
     * {
     *     return dividend / divisor;
     * }
     */
 
     /* 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 :)
      */
     //
     //加入新的样本
     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.
  */
 //TODO ack静默期间，提供额外的inflight数据
 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;
 
     // bbr->round_start不可能为1 ?
     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 ?
                            : 
             bbr->extra_acked[bbr->extra_acked_win_idx] = 0;
         }
     }
 
     /* Compute how many packets we expected to be delivered over epoch. */
     //计算期望发出的包
     //预期的ack = bbr_bw(sk) * (tp->delivered_mstamp - bbr->ack_epoch_mstamp) / BW_UNIT
     //static inline u32 tcp_stamp_us_delta(u64 t1, u64 t0)
     //{
     //    return max_t(s64, t1 - t0, 0);
     //}
     // bbr_bw(sk) : bbr->lt_use_bw == BBR_DRAIN ? bbr->lt_bw : bbr_max_bw(sk)
     epoch_us = tcp_stamp_us_delta(tp->delivered_mstamp,
                       bbr->ack_epoch_mstamp);    //epoch_us : 交付时间 - 采样开始的时间
     expected_acked = ((u64)bbr_bw(sk) * epoch_us) / BW_UNIT;    //预计的ack
 
     /* Reset the aggregation epoch if ACK rate is below expected rate or
      * significantly large no. of ack received since epoch (potentially
      * quite old epoch).
      */
     //重置周期
     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. */
     //计算超出预期的多余数据
     // extra_acked = min(bbr->ack_epoch_acked + rs->acked_sacked - expected_acked, tp->snd_cwnd)
     bbr->ack_epoch_acked = min_t(u32, 0xFFFFF,    //防止溢出（ack_epoch_acked：20）
                      bbr->ack_epoch_acked + rs->acked_sacked);    //ack + sack
     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])    //维护extra_acked最大值
         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.
  */
 //TODO 估计管道满时的情况 
 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;
     // bbr_full_bw_reached ： bbr->full_bw_reached 
     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;
     //更新full_bw full_bw_reached full_bw_cnt
     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. */
 //TODO 如果管道可能已满，则排空队列，然后进入稳定状态
 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. :-)
  */
 //TODO 更新最小RTT
 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;
}

//TODO 根据所处阶段更新gains
static void bbr_update_gains(struct sock *sk)
{
    struct bbr *bbr = inet_csk_ca(sk);

    switch (bbr->mode) {
    case BBR_STARTUP:
        // 739 739
        bbr->pacing_gain = bbr_high_gain;
        bbr->cwnd_gain     = bbr_high_gain;
        break;
    case BBR_DRAIN:
        //88 739
        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:
        // UNIT UNIT
        bbr->pacing_gain = BBR_UNIT;
        bbr->cwnd_gain     = BBR_UNIT;
        break;
    default:
        WARN_ONCE(1, "BBR bad mode: %u\n", bbr->mode);
        break;
    }
}

static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
{
    bbr_update_bw(sk, rs);                //估计带宽
    bbr_update_ack_aggregation(sk, rs);    //ack静默期间计算额外的inflight
    bbr_update_cycle_phase(sk, rs);        //增益循环
    bbr_check_full_bw_reached(sk, rs);    //估计管道满的情况
    bbr_check_drain(sk, rs);            //如果管道可能已满，则排空队列，然后进入稳定状态 
    bbr_update_min_rtt(sk, rs);            //更新最小rtt
    bbr_update_gains(sk);                //更新gains
}

//TODO main
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);                    //设置速度
    bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);        //设置cwnd
}

//TODO: 初始化
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,                 //初始化
    .cong_control    = bbr_main,             //main函数
    .sndbuf_expand    = bbr_sndbuf_expand,    //send缓冲区变化
    .undo_cwnd    = bbr_undo_cwnd,            //撤销cwnd（不需要）
    .cwnd_event    = bbr_cwnd_event,           //保存cwnd的值防止丢失
    .ssthresh    = bbr_ssthresh,             //设置拥塞阚值
    .min_tso_segs    = bbr_min_tso_segs,     //覆盖sysctl的tcp_min_tso_segs
    .get_info    = bbr_get_info,             //获取info
    .set_state    = bbr_set_state,            //设置状态，只有状态为TCP_CA_Loss时起作用
};  

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);