本文主要是介绍TCP拥塞控制算法BBR源码分析，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
BBR是谷歌与2016年提出的TCP拥塞控制算法，在Linux4.9的patch中正式加入。该算法一出，瞬间引起了极大的轰动。在CSDN上也有众多大佬对此进行分析讨论，褒贬不一。
本文首先对源码进行了分析，并在此基础上对BBR算法进行总结。
1.源码分析
/* 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)** pacing_rate和cwnd是整个算法最关键的核心所在，他们随着状态的变化而改变，并以此在实际上控制TCP的发包** 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.** Without fq qdisc, there may be some problem about RTT fair when lots of BBR* flows share a network but with different RTT.* Long RTT may hold more throughput than little one.* 详情可参考相关论文介绍：BBQ算法**/
#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)/*个人推测这里的BBR单位意思是使用kb作为单位，不知道理解的是否正确*/
#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四种标准状态*/
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? */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_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 */
};#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;/* 10s未更新最小RTT则进入PROBE_RTT* 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:** 模拟cubic的增加曲线做出的增长系数，这里类似于慢增长算法*/
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: */
/* 第一个RTT时间多发送四分之一，第二次少发送四分之一以排空队列，之后以估计窗口值发送6次，作为一整个循环*/
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:** 至少4个而不是1个是因为考虑到以下因素* （1）可能会有ACK延迟累积发送机制存在* （2）往返各2各则一共至少4个*/
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: * 论文中丢包率大于20%会有暴跌，就是这里带来的*/
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?检测STARTUP是否结束 */
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. 最大探测带宽*/
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. 设置估计带宽为LT_bw或者最大探测带宽*/
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;
}/* Convert a BBR bw and gain factor to a pacing rate in bytes per second. */
static u32 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;
}/* 初始化pacing 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);
}/* pacing_rate是控制速率的关键手段* 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 tcp_sock *tp = tcp_sk(sk);struct bbr *bbr = inet_csk_ca(sk);u32 rate = bbr_bw_to_pacing_rate(sk, bw, gain);/*如果未收到ACK则调用初始化速率*/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;
}/* 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 保存上次使用的拥塞窗口*/
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);
}/*拥塞窗口事件触发：如果在探测阶段则设置pacing rate*/
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 核心公式** 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* 此处解释为啥最少需要4个包 * 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; /* 初始值10 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;
}/* 保存窗口，方便从PROBE_RTT进入时恢复* 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 =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 = 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 */
}/*PROBE_RTT结束后的状态重置*/
static void bbr_reset_mode(struct sock *sk)
{if (!bbr_full_bw_reached(sk))bbr_reset_startup_mode(sk);elsebbr_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 = (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. * 通过三轮未增加带宽检测* 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.* * 第一轮接收窗口探测到了带宽的增加并增加窗口* 第二轮填满接收窗口* 第三轮返回高的传输速率*/
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;
}/* STARTUP后期，检查管道是否满了，满了则切换至DRAIN * 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 */
}/* PROBE_RTT状态* 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. :-)** 若在PROBE_RTT结束时，根据当前网络状况决定进入STARTUP还是PROBE_BW*/
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)) {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->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_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 &&after(tcp_jiffies32, bbr->probe_rtt_done_stamp)) {bbr->min_rtt_stamp = tcp_jiffies32;bbr->restore_cwnd = 1; /* snap to prior_cwnd */bbr_reset_mode(sk);}}}bbr->idle_restart = 0;
}/*全状态更新函数如下所列*/
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);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_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->restore_cwnd = 0;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);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_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)");
2.小结
BBR算法的特色在于摒弃了30年以来旧有的框架，是一种虽然不是全新但是很多年来未有人继续研究的新的框架。在CSDN和知乎上有不少很精彩的评论分析贴，不推荐看CSDN上某大神的大量文献资料（搜BBR最容易搜到的大神），因为个人感情色彩过于强烈、表述也有些条例上的问题，不过图做的很棒。知乎上有一篇关于BBR算法优势的帖子非常值得一学。当然，最值得学习的还是源码和论文。（这里不得不吐槽谷歌小哥的论文写的真的是相当的烂）
个人认为BBR算法值得去学习、改良，因为这种控制面和数据面解耦、不敏感于丢包的算法其实是TCP一大改进方向。总觉得TCP和UDP的改进最后应该是趋于互相学习优点剔除缺点，而BBR可以说是在这方面进了一大步。
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