diff --git a/include/uapi/linux/inet_diag.h b/include/uapi/linux/inet_diag.h
index b5c366f..509cd96 100644
--- a/include/uapi/linux/inet_diag.h
+++ b/include/uapi/linux/inet_diag.h
@@ -124,6 +124,7 @@ enum {
 	INET_DIAG_PEERS,
 	INET_DIAG_PAD,
 	INET_DIAG_MARK,
+	INET_DIAG_BBRINFO,
 	__INET_DIAG_MAX,
 };
 
@@ -157,8 +158,20 @@ struct tcp_dctcp_info {
 	__u32	dctcp_ab_tot;
 };
 
+/* INET_DIAG_BBRINFO */
+
+struct tcp_bbr_info {
+	/* u64 bw: max-filtered BW (app throughput) estimate in Byte per sec: */
+	__u32	bbr_bw_lo;		/* lower 32 bits of bw */
+	__u32	bbr_bw_hi;		/* upper 32 bits of bw */
+	__u32	bbr_min_rtt;		/* min-filtered RTT in uSec */
+	__u32	bbr_pacing_gain;	/* pacing gain shifted left 8 bits */
+	__u32	bbr_cwnd_gain;		/* cwnd gain shifted left 8 bits */
+};
+
 union tcp_cc_info {
 	struct tcpvegas_info	vegas;
 	struct tcp_dctcp_info	dctcp;
+	struct tcp_bbr_info	bbr;
 };
 #endif /* _UAPI_INET_DIAG_H_ */
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index 50d6a9b..300b068 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -640,6 +640,21 @@ config TCP_CONG_CDG
 	  D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
 	  delay gradients." In Networking 2011. Preprint: http://goo.gl/No3vdg
 
+config TCP_CONG_BBR
+	tristate "BBR TCP"
+	default n
+	---help---
+
+	BBR (Bottleneck Bandwidth and RTT) TCP congestion control aims to
+	maximize network utilization and minimize queues. It builds an explicit
+	model of the the bottleneck delivery rate and path round-trip
+	propagation delay. It tolerates packet loss and delay unrelated to
+	congestion. It can operate over LAN, WAN, cellular, wifi, or cable
+	modem links. It can coexist with flows that use loss-based congestion
+	control, and can operate with shallow buffers, deep buffers,
+	bufferbloat, policers, or AQM schemes that do not provide a delay
+	signal. It requires the fq ("Fair Queue") pacing packet scheduler.
+
 choice
 	prompt "Default TCP congestion control"
 	default DEFAULT_CUBIC
@@ -674,6 +689,9 @@ choice
 	config DEFAULT_CDG
 		bool "CDG" if TCP_CONG_CDG=y
 
+	config DEFAULT_BBR
+		bool "BBR" if TCP_CONG_BBR=y
+
 	config DEFAULT_RENO
 		bool "Reno"
 endchoice
diff --git a/net/ipv4/Makefile b/net/ipv4/Makefile
index 9cfff1a..bc6a6c8 100644
--- a/net/ipv4/Makefile
+++ b/net/ipv4/Makefile
@@ -41,6 +41,7 @@ obj-$(CONFIG_INET_DIAG) += inet_diag.o
 obj-$(CONFIG_INET_TCP_DIAG) += tcp_diag.o
 obj-$(CONFIG_INET_UDP_DIAG) += udp_diag.o
 obj-$(CONFIG_NET_TCPPROBE) += tcp_probe.o
+obj-$(CONFIG_TCP_CONG_BBR) += tcp_bbr.o
 obj-$(CONFIG_TCP_CONG_BIC) += tcp_bic.o
 obj-$(CONFIG_TCP_CONG_CDG) += tcp_cdg.o
 obj-$(CONFIG_TCP_CONG_CUBIC) += tcp_cubic.o
diff --git a/net/ipv4/tcp_bbr.c b/net/ipv4/tcp_bbr.c
new file mode 100644
index 0000000..34cde81
--- /dev/null
+++ b/net/ipv4/tcp_bbr.c
@@ -0,0 +1,875 @@
+/* Bottleneck Bandwidth and RTT (BBR) congestion control
+ *
+ * BBR congestion control computes the sending rate based on the delivery
+ * rate (throughput) estimated from ACKs. In a nutshell:
+ *
+ *   On each ACK, update our model of the network path:
+ *      bottleneck_bandwidth = windowed_max(delivered / elapsed, 10 round trips)
+ *      min_rtt = windowed_min(rtt, 10 seconds)
+ *   pacing_rate = pacing_gain * bottleneck_bandwidth
+ *   cwnd = max(cwnd_gain * bottleneck_bandwidth * min_rtt, 4)
+ *
+ * The core algorithm does not react directly to packet losses or delays,
+ * although BBR may adjust the size of next send per ACK when loss is
+ * observed, or adjust the sending rate if it estimates there is a
+ * traffic policer, in order to keep the drop rate reasonable.
+ *
+ * BBR is described in detail in:
+ *   "BBR: Congestion-Based Congestion Control",
+ *   Neal Cardwell, Yuchung Cheng, C. Stephen Gunn, Soheil Hassas Yeganeh,
+ *   Van Jacobson. ACM Queue, Vol. 14 No. 5, September-October 2016.
+ *
+ * There is a public e-mail list for discussing BBR development and testing:
+ *   https://groups.google.com/forum/#!forum/bbr-dev
+ *
+ * NOTE: BBR *must* be used with the fq qdisc ("man tc-fq") with pacing enabled,
+ * since pacing is integral to the BBR design and implementation.
+ * BBR without pacing would not function properly, and may incur unnecessary
+ * high packet loss rates.
+ */
+#include <linux/module.h>
+#include <net/tcp.h>
+#include <linux/inet_diag.h>
+#include <linux/inet.h>
+#include <linux/random.h>
+#include <linux/win_minmax.h>
+
+/* Scale factor for rate in pkt/uSec unit to avoid truncation in bandwidth
+ * estimation. The rate unit ~= (1500 bytes / 1 usec / 2^24) ~= 715 bps.
+ * This handles bandwidths from 0.06pps (715bps) to 256Mpps (3Tbps) in a u32.
+ * Since the minimum window is >=4 packets, the lower bound isn't
+ * an issue. The upper bound isn't an issue with existing technologies.
+ */
+#define BW_SCALE 24
+#define BW_UNIT (1 << BW_SCALE)
+
+#define BBR_SCALE 8	/* scaling factor for fractions in BBR (e.g. gains) */
+#define BBR_UNIT (1 << BBR_SCALE)
+
+/* BBR has the following modes for deciding how fast to send: */
+enum bbr_mode {
+	BBR_STARTUP,	/* ramp up sending rate rapidly to fill pipe */
+	BBR_DRAIN,	/* drain any queue created during startup */
+	BBR_PROBE_BW,	/* discover, share bw: pace around estimated bw */
+	BBR_PROBE_RTT,	/* cut cwnd to min to probe min_rtt */
+};
+
+/* BBR congestion control block */
+struct bbr {
+	u32	min_rtt_us;	        /* min RTT in min_rtt_win_sec window */
+	u32	min_rtt_stamp;	        /* timestamp of min_rtt_us */
+	u32	probe_rtt_done_stamp;   /* end time for BBR_PROBE_RTT mode */
+	struct minmax bw;	/* Max recent delivery rate in pkts/uS << 24 */
+	u32	rtt_cnt;	    /* count of packet-timed rounds elapsed */
+	u32     next_rtt_delivered; /* scb->tx.delivered at end of round */
+	struct skb_mstamp cycle_mstamp;  /* time of this cycle phase start */
+	u32     mode:3,		     /* current bbr_mode in state machine */
+		prev_ca_state:3,     /* CA state on previous ACK */
+		packet_conservation:1,  /* use packet conservation? */
+		restore_cwnd:1,	     /* decided to revert cwnd to old value */
+		round_start:1,	     /* start of packet-timed tx->ack round? */
+		tso_segs_goal:7,     /* segments we want in each skb we send */
+		idle_restart:1,	     /* restarting after idle? */
+		probe_rtt_round_done:1,  /* a BBR_PROBE_RTT round at 4 pkts? */
+		unused:5,
+		lt_is_sampling:1,    /* taking long-term ("LT") samples now? */
+		lt_rtt_cnt:7,	     /* round trips in long-term interval */
+		lt_use_bw:1;	     /* use lt_bw as our bw estimate? */
+	u32	lt_bw;		     /* LT est delivery rate in pkts/uS << 24 */
+	u32	lt_last_delivered;   /* LT intvl start: tp->delivered */
+	u32	lt_last_stamp;	     /* LT intvl start: tp->delivered_mstamp */
+	u32	lt_last_lost;	     /* LT intvl start: tp->lost */
+	u32	pacing_gain:10,	/* current gain for setting pacing rate */
+		cwnd_gain:10,	/* current gain for setting cwnd */
+		full_bw_cnt:3,	/* number of rounds without large bw gains */
+		cycle_idx:3,	/* current index in pacing_gain cycle array */
+		unused_b:6;
+	u32	prior_cwnd;	/* prior cwnd upon entering loss recovery */
+	u32	full_bw;	/* recent bw, to estimate if pipe is full */
+};
+
+#define CYCLE_LEN	8	/* number of phases in a pacing gain cycle */
+
+static int bbr_bw_rtts	= CYCLE_LEN + 2; /* win len of bw filter (in rounds) */
+static u32 bbr_min_rtt_win_sec = 10;	 /* min RTT filter window (in sec) */
+static u32 bbr_probe_rtt_mode_ms = 200;	 /* min ms at cwnd=4 in BBR_PROBE_RTT */
+static int bbr_min_tso_rate	= 1200000;  /* skip TSO below here (bits/sec) */
+
+/* We use a high_gain value chosen to 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 int bbr_high_gain  = BBR_UNIT * 2885 / 1000 + 1;	/* 2/ln(2) */
+static int bbr_drain_gain = BBR_UNIT * 1000 / 2885;	/* 1/high_gain */
+static int bbr_cwnd_gain  = BBR_UNIT * 2;	/* gain for steady-state cwnd */
+/* The pacing_gain values for the PROBE_BW gain cycle: */
+static int bbr_pacing_gain[] = { BBR_UNIT * 5 / 4, BBR_UNIT * 3 / 4,
+				 BBR_UNIT, BBR_UNIT, BBR_UNIT,
+				 BBR_UNIT, BBR_UNIT, BBR_UNIT };
+static u32 bbr_cycle_rand = 7;  /* randomize gain cycling phase over N phases */
+
+/* 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 u32 bbr_cwnd_min_target	= 4;
+
+/* To estimate if BBR_STARTUP mode (i.e. high_gain) has filled pipe. */
+static u32 bbr_full_bw_thresh = BBR_UNIT * 5 / 4;  /* bw up 1.25x per round? */
+static u32 bbr_full_bw_cnt    = 3;    /* N rounds w/o bw growth -> pipe full */
+
+/* "long-term" ("LT") bandwidth estimator parameters: */
+static bool bbr_lt_bw_estimator = true;	/* use the long-term bw estimate? */
+static u32 bbr_lt_intvl_min_rtts = 4;	/* min rounds in sampling interval */
+static u32 bbr_lt_loss_thresh = 50;	/*  lost/delivered > 20% -> "lossy" */
+static u32 bbr_lt_conv_thresh = BBR_UNIT / 8;  /* bw diff <= 12.5% -> "close" */
+static u32 bbr_lt_bw_max_rtts	= 48;	/* max # of round trips using lt_bw */
+
+/* Do we estimate that STARTUP filled the pipe? */
+static bool bbr_full_bw_reached(const struct sock *sk)
+{
+	const struct bbr *bbr = inet_csk_ca(sk);
+
+	return bbr->full_bw_cnt >= bbr_full_bw_cnt;
+}
+
+/* Return the windowed max recent bandwidth sample, in pkts/uS << BW_SCALE. */
+static u32 bbr_max_bw(const struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	return minmax_get(&bbr->bw);
+}
+
+/* Return the estimated bandwidth of the path, in pkts/uS << BW_SCALE. */
+static u32 bbr_bw(const struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	return bbr->lt_use_bw ? bbr->lt_bw : bbr_max_bw(sk);
+}
+
+/* Return rate in bytes per second, optionally with a gain.
+ * The order here is chosen carefully to avoid overflow of u64. This should
+ * work for input rates of up to 2.9Tbit/sec and gain of 2.89x.
+ */
+static u64 bbr_rate_bytes_per_sec(struct sock *sk, u64 rate, int gain)
+{
+	rate *= tcp_mss_to_mtu(sk, tcp_sk(sk)->mss_cache);
+	rate *= gain;
+	rate >>= BBR_SCALE;
+	rate *= USEC_PER_SEC;
+	return rate >> BW_SCALE;
+}
+
+static u64 bbr_rate_kbps(struct sock *sk, u64 rate)
+{
+	return bbr_rate_bytes_per_sec(sk, rate, BBR_UNIT) * 8 / 1000;
+}
+
+/* Pace using current bw estimate and a gain factor. */
+static void bbr_set_pacing_rate(struct sock *sk, u32 bw, int gain)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+	u64 rate = bw;
+
+	rate = bbr_rate_bytes_per_sec(sk, rate, gain);
+	rate = min_t(u64, rate, sk->sk_max_pacing_rate);
+	if (bbr->mode != BBR_STARTUP || rate > sk->sk_pacing_rate)
+		sk->sk_pacing_rate = rate;
+}
+
+/* Return count of segments we want in the skbs we send, or 0 for default. */
+static u32 bbr_tso_segs_goal(struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	return bbr->tso_segs_goal;
+}
+
+static void bbr_set_tso_segs_goal(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 min_segs;
+
+	min_segs = sk->sk_pacing_rate < (bbr_min_tso_rate >> 3) ? 1 : 2;
+	bbr->tso_segs_goal = min(tcp_tso_autosize(sk, tp->mss_cache, min_segs),
+				 0x7FU);
+}
+
+/* Save "last known good" cwnd so we can restore it after losses or PROBE_RTT */
+static void bbr_save_cwnd(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	if (bbr->prev_ca_state < TCP_CA_Recovery && bbr->mode != BBR_PROBE_RTT)
+		bbr->prior_cwnd = tp->snd_cwnd;  /* this cwnd is good enough */
+	else  /* loss recovery or BBR_PROBE_RTT have temporarily cut cwnd */
+		bbr->prior_cwnd = max(bbr->prior_cwnd, tp->snd_cwnd);
+}
+
+static void bbr_cwnd_event(struct sock *sk, enum tcp_ca_event event)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	if (event == CA_EVENT_TX_START && tp->app_limited) {
+		bbr->idle_restart = 1;
+		/* Avoid pointless buffer overflows: pace at est. bw if we don't
+		 * need more speed (we're restarting from idle and app-limited).
+		 */
+		if (bbr->mode == BBR_PROBE_BW)
+			bbr_set_pacing_rate(sk, bbr_bw(sk), BBR_UNIT);
+	}
+}
+
+/* Find target cwnd. Right-size the cwnd based on min RTT and the
+ * estimated bottleneck bandwidth:
+ *
+ * cwnd = bw * min_rtt * gain = BDP * gain
+ *
+ * The key factor, gain, controls the amount of queue. While a small gain
+ * builds a smaller queue, it becomes more vulnerable to noise in RTT
+ * measurements (e.g., delayed ACKs or other ACK compression effects). This
+ * noise may cause BBR to under-estimate the rate.
+ *
+ * To achieve full performance in high-speed paths, we budget enough cwnd to
+ * fit full-sized skbs in-flight on both end hosts to fully utilize the path:
+ *   - one skb in sending host Qdisc,
+ *   - one skb in sending host TSO/GSO engine
+ *   - one skb being received by receiver host LRO/GRO/delayed-ACK engine
+ * Don't worry, at low rates (bbr_min_tso_rate) this won't bloat cwnd because
+ * in such cases tso_segs_goal is 1. The minimum cwnd is 4 packets,
+ * which allows 2 outstanding 2-packet sequences, to try to keep pipe
+ * full even with ACK-every-other-packet delayed ACKs.
+ */
+static u32 bbr_target_cwnd(struct sock *sk, u32 bw, int gain)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 cwnd;
+	u64 w;
+
+	/* If we've never had a valid RTT sample, cap cwnd at the initial
+	 * default. This should only happen when the connection is not using TCP
+	 * timestamps and has retransmitted all of the SYN/SYNACK/data packets
+	 * ACKed so far. In this case, an RTO can cut cwnd to 1, in which
+	 * case we need to slow-start up toward something safe: TCP_INIT_CWND.
+	 */
+	if (unlikely(bbr->min_rtt_us == ~0U))	 /* no valid RTT samples yet? */
+		return TCP_INIT_CWND;  /* be safe: cap at default initial cwnd*/
+
+	w = (u64)bw * bbr->min_rtt_us;
+
+	/* Apply a gain to the given value, then remove the BW_SCALE shift. */
+	cwnd = (((w * gain) >> BBR_SCALE) + BW_UNIT - 1) / BW_UNIT;
+
+	/* Allow enough full-sized skbs in flight to utilize end systems. */
+	cwnd += 3 * bbr->tso_segs_goal;
+
+	/* Reduce delayed ACKs by rounding up cwnd to the next even number. */
+	cwnd = (cwnd + 1) & ~1U;
+
+	return cwnd;
+}
+
+/* An optimization in BBR to reduce losses: On the first round of recovery, we
+ * follow the packet conservation principle: send P packets per P packets acked.
+ * After that, we slow-start and send at most 2*P packets per P packets acked.
+ * After recovery finishes, or upon undo, we restore the cwnd we had when
+ * recovery started (capped by the target cwnd based on estimated BDP).
+ *
+ * TODO(ycheng/ncardwell): implement a rate-based approach.
+ */
+static bool bbr_set_cwnd_to_recover_or_restore(
+	struct sock *sk, const struct rate_sample *rs, u32 acked, u32 *new_cwnd)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u8 prev_state = bbr->prev_ca_state, state = inet_csk(sk)->icsk_ca_state;
+	u32 cwnd = tp->snd_cwnd;
+
+	/* An ACK for P pkts should release at most 2*P packets. We do this
+	 * in two steps. First, here we deduct the number of lost packets.
+	 * Then, in bbr_set_cwnd() we slow start up toward the target cwnd.
+	 */
+	if (rs->losses > 0)
+		cwnd = max_t(s32, cwnd - rs->losses, 1);
+
+	if (state == TCP_CA_Recovery && prev_state != TCP_CA_Recovery) {
+		/* Starting 1st round of Recovery, so do packet conservation. */
+		bbr->packet_conservation = 1;
+		bbr->next_rtt_delivered = tp->delivered;  /* start round now */
+		/* Cut unused cwnd from app behavior, TSQ, or TSO deferral: */
+		cwnd = tcp_packets_in_flight(tp) + acked;
+	} else if (prev_state >= TCP_CA_Recovery && state < TCP_CA_Recovery) {
+		/* Exiting loss recovery; restore cwnd saved before recovery. */
+		bbr->restore_cwnd = 1;
+		bbr->packet_conservation = 0;
+	}
+	bbr->prev_ca_state = state;
+
+	if (bbr->restore_cwnd) {
+		/* Restore cwnd after exiting loss recovery or PROBE_RTT. */
+		cwnd = max(cwnd, bbr->prior_cwnd);
+		bbr->restore_cwnd = 0;
+	}
+
+	if (bbr->packet_conservation) {
+		*new_cwnd = max(cwnd, tcp_packets_in_flight(tp) + acked);
+		return true;	/* yes, using packet conservation */
+	}
+	*new_cwnd = cwnd;
+	return false;
+}
+
+/* Slow-start up toward target cwnd (if bw estimate is growing, or packet loss
+ * has drawn us down below target), or snap down to target if we're above it.
+ */
+static void bbr_set_cwnd(struct sock *sk, const struct rate_sample *rs,
+			 u32 acked, u32 bw, int gain)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 cwnd = 0, target_cwnd = 0;
+
+	if (!acked)
+		return;
+
+	if (bbr_set_cwnd_to_recover_or_restore(sk, rs, acked, &cwnd))
+		goto done;
+
+	/* If we're below target cwnd, slow start cwnd toward target cwnd. */
+	target_cwnd = bbr_target_cwnd(sk, bw, gain);
+	if (bbr_full_bw_reached(sk))  /* only cut cwnd if we filled the pipe */
+		cwnd = min(cwnd + acked, target_cwnd);
+	else if (cwnd < target_cwnd || tp->delivered < TCP_INIT_CWND)
+		cwnd = cwnd + acked;
+	cwnd = max(cwnd, bbr_cwnd_min_target);
+
+done:
+	tp->snd_cwnd = min(cwnd, tp->snd_cwnd_clamp);	/* apply global cap */
+	if (bbr->mode == BBR_PROBE_RTT)  /* drain queue, refresh min_rtt */
+		tp->snd_cwnd = min(tp->snd_cwnd, bbr_cwnd_min_target);
+}
+
+/* End cycle phase if it's time and/or we hit the phase's in-flight target. */
+static bool bbr_is_next_cycle_phase(struct sock *sk,
+				    const struct rate_sample *rs)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	bool is_full_length =
+		skb_mstamp_us_delta(&tp->delivered_mstamp, &bbr->cycle_mstamp) >
+		bbr->min_rtt_us;
+	u32 inflight, bw;
+
+	/* The pacing_gain of 1.0 paces at the estimated bw to try to fully
+	 * use the pipe without increasing the queue.
+	 */
+	if (bbr->pacing_gain == BBR_UNIT)
+		return is_full_length;		/* just use wall clock time */
+
+	inflight = rs->prior_in_flight;  /* what was in-flight before ACK? */
+	bw = bbr_max_bw(sk);
+
+	/* A pacing_gain > 1.0 probes for bw by trying to raise inflight to at
+	 * least pacing_gain*BDP; this may take more than min_rtt if min_rtt is
+	 * small (e.g. on a LAN). We do not persist if packets are lost, since
+	 * a path with small buffers may not hold that much.
+	 */
+	if (bbr->pacing_gain > BBR_UNIT)
+		return is_full_length &&
+			(rs->losses ||  /* perhaps pacing_gain*BDP won't fit */
+			 inflight >= bbr_target_cwnd(sk, bw, bbr->pacing_gain));
+
+	/* A pacing_gain < 1.0 tries to drain extra queue we added if bw
+	 * probing didn't find more bw. If inflight falls to match BDP then we
+	 * estimate queue is drained; persisting would underutilize the pipe.
+	 */
+	return is_full_length ||
+		inflight <= bbr_target_cwnd(sk, bw, BBR_UNIT);
+}
+
+static void bbr_advance_cycle_phase(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	bbr->cycle_idx = (bbr->cycle_idx + 1) & (CYCLE_LEN - 1);
+	bbr->cycle_mstamp = tp->delivered_mstamp;
+	bbr->pacing_gain = bbr_pacing_gain[bbr->cycle_idx];
+}
+
+/* Gain cycling: cycle pacing gain to converge to fair share of available bw. */
+static void bbr_update_cycle_phase(struct sock *sk,
+				   const struct rate_sample *rs)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	if ((bbr->mode == BBR_PROBE_BW) && !bbr->lt_use_bw &&
+	    bbr_is_next_cycle_phase(sk, rs))
+		bbr_advance_cycle_phase(sk);
+}
+
+static void bbr_reset_startup_mode(struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	bbr->mode = BBR_STARTUP;
+	bbr->pacing_gain = bbr_high_gain;
+	bbr->cwnd_gain	 = bbr_high_gain;
+}
+
+static void bbr_reset_probe_bw_mode(struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	bbr->mode = BBR_PROBE_BW;
+	bbr->pacing_gain = BBR_UNIT;
+	bbr->cwnd_gain = bbr_cwnd_gain;
+	bbr->cycle_idx = CYCLE_LEN - 1 - prandom_u32_max(bbr_cycle_rand);
+	bbr_advance_cycle_phase(sk);	/* flip to next phase of gain cycle */
+}
+
+static void bbr_reset_mode(struct sock *sk)
+{
+	if (!bbr_full_bw_reached(sk))
+		bbr_reset_startup_mode(sk);
+	else
+		bbr_reset_probe_bw_mode(sk);
+}
+
+/* Start a new long-term sampling interval. */
+static void bbr_reset_lt_bw_sampling_interval(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	bbr->lt_last_stamp = tp->delivered_mstamp.stamp_jiffies;
+	bbr->lt_last_delivered = tp->delivered;
+	bbr->lt_last_lost = tp->lost;
+	bbr->lt_rtt_cnt = 0;
+}
+
+/* Completely reset long-term bandwidth sampling. */
+static void bbr_reset_lt_bw_sampling(struct sock *sk)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	bbr->lt_bw = 0;
+	bbr->lt_use_bw = 0;
+	bbr->lt_is_sampling = false;
+	bbr_reset_lt_bw_sampling_interval(sk);
+}
+
+/* Long-term bw sampling interval is done. Estimate whether we're policed. */
+static void bbr_lt_bw_interval_done(struct sock *sk, u32 bw)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 diff;
+
+	if (bbr->lt_bw &&  /* do we have bw from a previous interval? */
+	    bbr_lt_bw_estimator) {  /* using long-term bw estimator enabled? */
+		/* Is new bw close to the lt_bw from the previous interval? */
+		diff = abs(bw - bbr->lt_bw);
+		if ((diff * BBR_UNIT <= bbr_lt_conv_thresh * bbr->lt_bw) ||
+		    (bbr_rate_kbps(sk, diff) <= 4)) {  /* diff <= 4 Kbit/sec? */
+			/* All criteria are met; estimate we're policed. */
+			bbr->lt_bw = (bw + bbr->lt_bw) >> 1;  /* avg 2 intvls */
+			bbr->lt_use_bw = 1;
+			bbr->pacing_gain = BBR_UNIT;  /* try to avoid drops */
+			bbr->lt_rtt_cnt = 0;
+			return;
+		}
+	}
+	bbr->lt_bw = bw;
+	bbr_reset_lt_bw_sampling_interval(sk);
+}
+
+/* Token-bucket traffic policers are common (see "An Internet-Wide Analysis of
+ * Traffic Policing", SIGCOMM 2016). BBR detects token-bucket policers and
+ * explicitly models their policed rate, to reduce unnecessary losses. We
+ * estimate that we're policed if we see 2 consecutive sampling intervals with
+ * consistent throughput and high packet loss. If we think we're being policed,
+ * set lt_bw to the "long-term" average delivery rate from those 2 intervals.
+ */
+static void bbr_lt_bw_sampling(struct sock *sk, const struct rate_sample *rs)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 lost, delivered;
+	u64 bw;
+	s32 t;
+
+	if (bbr->lt_use_bw) {	/* already using long-term rate, lt_bw? */
+		if (bbr->mode == BBR_PROBE_BW && bbr->round_start &&
+		    ++bbr->lt_rtt_cnt >= bbr_lt_bw_max_rtts) {
+			bbr_reset_lt_bw_sampling(sk);    /* stop using lt_bw */
+			bbr_reset_probe_bw_mode(sk);  /* restart gain cycling */
+		}
+		return;
+	}
+
+	/* Wait for the first loss before sampling, to let the policer exhaust
+	 * its tokens and estimate the steady-state rate allowed by the policer.
+	 * Starting samples earlier includes bursts that over-estimate the bw.
+	 */
+	if (!bbr->lt_is_sampling) {
+		if (!rs->losses)
+			return;
+		bbr_reset_lt_bw_sampling_interval(sk);
+		bbr->lt_is_sampling = true;
+	}
+
+	/* To avoid underestimates, reset sampling if we run out of data. */
+	if (rs->is_app_limited) {
+		bbr_reset_lt_bw_sampling(sk);
+		return;
+	}
+
+	if (bbr->round_start)
+		bbr->lt_rtt_cnt++;	/* count round trips in this interval */
+	if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts)
+		return;		/* sampling interval needs to be longer */
+	if (bbr->lt_rtt_cnt > 4 * bbr_lt_intvl_min_rtts) {
+		bbr_reset_lt_bw_sampling(sk);  /* interval is too long */
+		return;
+	}
+
+	/* End sampling interval when a packet is lost, so we estimate the
+	 * policer tokens were exhausted. Stopping the sampling before the
+	 * tokens are exhausted under-estimates the policed rate.
+	 */
+	if (!rs->losses)
+		return;
+
+	/* Calculate packets lost and delivered in sampling interval. */
+	lost = tp->lost - bbr->lt_last_lost;
+	delivered = tp->delivered - bbr->lt_last_delivered;
+	/* Is loss rate (lost/delivered) >= lt_loss_thresh? If not, wait. */
+	if (!delivered || (lost << BBR_SCALE) < bbr_lt_loss_thresh * delivered)
+		return;
+
+	/* Find average delivery rate in this sampling interval. */
+	t = (s32)(tp->delivered_mstamp.stamp_jiffies - bbr->lt_last_stamp);
+	if (t < 1)
+		return;		/* interval is less than one jiffy, so wait */
+	t = jiffies_to_usecs(t);
+	/* Interval long enough for jiffies_to_usecs() to return a bogus 0? */
+	if (t < 1) {
+		bbr_reset_lt_bw_sampling(sk);  /* interval too long; reset */
+		return;
+	}
+	bw = (u64)delivered * BW_UNIT;
+	do_div(bw, t);
+	bbr_lt_bw_interval_done(sk, bw);
+}
+
+/* Estimate the bandwidth based on how fast packets are delivered */
+static void bbr_update_bw(struct sock *sk, const struct rate_sample *rs)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u64 bw;
+
+	bbr->round_start = 0;
+	if (rs->delivered < 0 || rs->interval_us <= 0)
+		return; /* Not a valid observation */
+
+	/* See if we've reached the next RTT */
+	if (!before(rs->prior_delivered, bbr->next_rtt_delivered)) {
+		bbr->next_rtt_delivered = tp->delivered;
+		bbr->rtt_cnt++;
+		bbr->round_start = 1;
+		bbr->packet_conservation = 0;
+	}
+
+	bbr_lt_bw_sampling(sk, rs);
+
+	/* Divide delivered by the interval to find a (lower bound) bottleneck
+	 * bandwidth sample. Delivered is in packets and interval_us in uS and
+	 * ratio will be <<1 for most connections. So delivered is first scaled.
+	 */
+	bw = (u64)rs->delivered * BW_UNIT;
+	do_div(bw, rs->interval_us);
+
+	/* If this sample is application-limited, it is likely to have a very
+	 * low delivered count that represents application behavior rather than
+	 * the available network rate. Such a sample could drag down estimated
+	 * bw, causing needless slow-down. Thus, to continue to send at the
+	 * last measured network rate, we filter out app-limited samples unless
+	 * they describe the path bw at least as well as our bw model.
+	 *
+	 * So the goal during app-limited phase is to proceed with the best
+	 * network rate no matter how long. We automatically leave this
+	 * phase when app writes faster than the network can deliver :)
+	 */
+	if (!rs->is_app_limited || bw >= bbr_max_bw(sk)) {
+		/* Incorporate new sample into our max bw filter. */
+		minmax_running_max(&bbr->bw, bbr_bw_rtts, bbr->rtt_cnt, bw);
+	}
+}
+
+/* Estimate when the pipe is full, using the change in delivery rate: BBR
+ * estimates that STARTUP filled the pipe if the estimated bw hasn't changed by
+ * at least bbr_full_bw_thresh (25%) after bbr_full_bw_cnt (3) non-app-limited
+ * rounds. Why 3 rounds: 1: rwin autotuning grows the rwin, 2: we fill the
+ * higher rwin, 3: we get higher delivery rate samples. Or transient
+ * cross-traffic or radio noise can go away. CUBIC Hystart shares a similar
+ * design goal, but uses delay and inter-ACK spacing instead of bandwidth.
+ */
+static void bbr_check_full_bw_reached(struct sock *sk,
+				      const struct rate_sample *rs)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 bw_thresh;
+
+	if (bbr_full_bw_reached(sk) || !bbr->round_start || rs->is_app_limited)
+		return;
+
+	bw_thresh = (u64)bbr->full_bw * bbr_full_bw_thresh >> BBR_SCALE;
+	if (bbr_max_bw(sk) >= bw_thresh) {
+		bbr->full_bw = bbr_max_bw(sk);
+		bbr->full_bw_cnt = 0;
+		return;
+	}
+	++bbr->full_bw_cnt;
+}
+
+/* If pipe is probably full, drain the queue and then enter steady-state. */
+static void bbr_check_drain(struct sock *sk, const struct rate_sample *rs)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	if (bbr->mode == BBR_STARTUP && bbr_full_bw_reached(sk)) {
+		bbr->mode = BBR_DRAIN;	/* drain queue we created */
+		bbr->pacing_gain = bbr_drain_gain;	/* pace slow to drain */
+		bbr->cwnd_gain = bbr_high_gain;	/* maintain cwnd */
+	}	/* fall through to check if in-flight is already small: */
+	if (bbr->mode == BBR_DRAIN &&
+	    tcp_packets_in_flight(tcp_sk(sk)) <=
+	    bbr_target_cwnd(sk, bbr_max_bw(sk), BBR_UNIT))
+		bbr_reset_probe_bw_mode(sk);  /* we estimate queue is drained */
+}
+
+/* The goal of PROBE_RTT mode is to have BBR flows cooperatively and
+ * periodically drain the bottleneck queue, to converge to measure the true
+ * min_rtt (unloaded propagation delay). This allows the flows to keep queues
+ * small (reducing queuing delay and packet loss) and achieve fairness among
+ * BBR flows.
+ *
+ * The min_rtt filter window is 10 seconds. When the min_rtt estimate expires,
+ * we enter PROBE_RTT mode and cap the cwnd at bbr_cwnd_min_target=4 packets.
+ * After at least bbr_probe_rtt_mode_ms=200ms and at least one packet-timed
+ * round trip elapsed with that flight size <= 4, we leave PROBE_RTT mode and
+ * re-enter the previous mode. BBR uses 200ms to approximately bound the
+ * performance penalty of PROBE_RTT's cwnd capping to roughly 2% (200ms/10s).
+ *
+ * Note that flows need only pay 2% if they are busy sending over the last 10
+ * seconds. Interactive applications (e.g., Web, RPCs, video chunks) often have
+ * natural silences or low-rate periods within 10 seconds where the rate is low
+ * enough for long enough to drain its queue in the bottleneck. We pick up
+ * these min RTT measurements opportunistically with our min_rtt filter. :-)
+ */
+static void bbr_update_min_rtt(struct sock *sk, const struct rate_sample *rs)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	bool filter_expired;
+
+	/* Track min RTT seen in the min_rtt_win_sec filter window: */
+	filter_expired = after(tcp_time_stamp,
+			       bbr->min_rtt_stamp + bbr_min_rtt_win_sec * HZ);
+	if (rs->rtt_us >= 0 &&
+	    (rs->rtt_us <= bbr->min_rtt_us || filter_expired)) {
+		bbr->min_rtt_us = rs->rtt_us;
+		bbr->min_rtt_stamp = tcp_time_stamp;
+	}
+
+	if (bbr_probe_rtt_mode_ms > 0 && filter_expired &&
+	    !bbr->idle_restart && bbr->mode != BBR_PROBE_RTT) {
+		bbr->mode = BBR_PROBE_RTT;  /* dip, drain queue */
+		bbr->pacing_gain = BBR_UNIT;
+		bbr->cwnd_gain = BBR_UNIT;
+		bbr_save_cwnd(sk);  /* note cwnd so we can restore it */
+		bbr->probe_rtt_done_stamp = 0;
+	}
+
+	if (bbr->mode == BBR_PROBE_RTT) {
+		/* Ignore low rate samples during this mode. */
+		tp->app_limited =
+			(tp->delivered + tcp_packets_in_flight(tp)) ? : 1;
+		/* Maintain min packets in flight for max(200 ms, 1 round). */
+		if (!bbr->probe_rtt_done_stamp &&
+		    tcp_packets_in_flight(tp) <= bbr_cwnd_min_target) {
+			bbr->probe_rtt_done_stamp = tcp_time_stamp +
+				msecs_to_jiffies(bbr_probe_rtt_mode_ms);
+			bbr->probe_rtt_round_done = 0;
+			bbr->next_rtt_delivered = tp->delivered;
+		} else if (bbr->probe_rtt_done_stamp) {
+			if (bbr->round_start)
+				bbr->probe_rtt_round_done = 1;
+			if (bbr->probe_rtt_round_done &&
+			    after(tcp_time_stamp, bbr->probe_rtt_done_stamp)) {
+				bbr->min_rtt_stamp = tcp_time_stamp;
+				bbr->restore_cwnd = 1;  /* snap to prior_cwnd */
+				bbr_reset_mode(sk);
+			}
+		}
+	}
+	bbr->idle_restart = 0;
+}
+
+static void bbr_update_model(struct sock *sk, const struct rate_sample *rs)
+{
+	bbr_update_bw(sk, rs);
+	bbr_update_cycle_phase(sk, rs);
+	bbr_check_full_bw_reached(sk, rs);
+	bbr_check_drain(sk, rs);
+	bbr_update_min_rtt(sk, rs);
+}
+
+static void bbr_main(struct sock *sk, const struct rate_sample *rs)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+	u32 bw;
+
+	bbr_update_model(sk, rs);
+
+	bw = bbr_bw(sk);
+	bbr_set_pacing_rate(sk, bw, bbr->pacing_gain);
+	bbr_set_tso_segs_goal(sk);
+	bbr_set_cwnd(sk, rs, rs->acked_sacked, bw, bbr->cwnd_gain);
+}
+
+static void bbr_init(struct sock *sk)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bbr *bbr = inet_csk_ca(sk);
+	u64 bw;
+
+	bbr->prior_cwnd = 0;
+	bbr->tso_segs_goal = 0;	 /* default segs per skb until first ACK */
+	bbr->rtt_cnt = 0;
+	bbr->next_rtt_delivered = 0;
+	bbr->prev_ca_state = TCP_CA_Open;
+	bbr->packet_conservation = 0;
+
+	bbr->probe_rtt_done_stamp = 0;
+	bbr->probe_rtt_round_done = 0;
+	bbr->min_rtt_us = tcp_min_rtt(tp);
+	bbr->min_rtt_stamp = tcp_time_stamp;
+
+	minmax_reset(&bbr->bw, bbr->rtt_cnt, 0);  /* init max bw to 0 */
+
+	/* Initialize pacing rate to: high_gain * init_cwnd / RTT. */
+	bw = (u64)tp->snd_cwnd * BW_UNIT;
+	do_div(bw, (tp->srtt_us >> 3) ? : USEC_PER_MSEC);
+	sk->sk_pacing_rate = 0;		/* force an update of sk_pacing_rate */
+	bbr_set_pacing_rate(sk, bw, bbr_high_gain);
+
+	bbr->restore_cwnd = 0;
+	bbr->round_start = 0;
+	bbr->idle_restart = 0;
+	bbr->full_bw = 0;
+	bbr->full_bw_cnt = 0;
+	bbr->cycle_mstamp.v64 = 0;
+	bbr->cycle_idx = 0;
+	bbr_reset_lt_bw_sampling(sk);
+	bbr_reset_startup_mode(sk);
+}
+
+static u32 bbr_sndbuf_expand(struct sock *sk)
+{
+	/* Provision 3 * cwnd since BBR may slow-start even during recovery. */
+	return 3;
+}
+
+/* In theory BBR does not need to undo the cwnd since it does not
+ * always reduce cwnd on losses (see bbr_main()). Keep it for now.
+ */
+static u32 bbr_undo_cwnd(struct sock *sk)
+{
+	return tcp_sk(sk)->snd_cwnd;
+}
+
+/* Entering loss recovery, so save cwnd for when we exit or undo recovery. */
+static u32 bbr_ssthresh(struct sock *sk)
+{
+	bbr_save_cwnd(sk);
+	return TCP_INFINITE_SSTHRESH;	 /* BBR does not use ssthresh */
+}
+
+static size_t bbr_get_info(struct sock *sk, u32 ext, int *attr,
+			   union tcp_cc_info *info)
+{
+	if (ext & (1 << (INET_DIAG_BBRINFO - 1)) ||
+	    ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
+		struct tcp_sock *tp = tcp_sk(sk);
+		struct bbr *bbr = inet_csk_ca(sk);
+		u64 bw = bbr_bw(sk);
+
+		bw = bw * tp->mss_cache * USEC_PER_SEC >> BW_SCALE;
+		memset(&info->bbr, 0, sizeof(info->bbr));
+		info->bbr.bbr_bw_lo		= (u32)bw;
+		info->bbr.bbr_bw_hi		= (u32)(bw >> 32);
+		info->bbr.bbr_min_rtt		= bbr->min_rtt_us;
+		info->bbr.bbr_pacing_gain	= bbr->pacing_gain;
+		info->bbr.bbr_cwnd_gain		= bbr->cwnd_gain;
+		*attr = INET_DIAG_BBRINFO;
+		return sizeof(info->bbr);
+	}
+	return 0;
+}
+
+static void bbr_set_state(struct sock *sk, u8 new_state)
+{
+	struct bbr *bbr = inet_csk_ca(sk);
+
+	if (new_state == TCP_CA_Loss) {
+		struct rate_sample rs = { .prior_mstamp.v64 = 0, .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("GPL");
+MODULE_DESCRIPTION("TCP BBR (Bottleneck Bandwidth and RTT)");
