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![Yuchung Cheng](/assets/img/avatar_default.png)
Slow start now increases cwnd by 1 if an ACK acknowledges some packets, regardless the number of packets. Consequently slow start performance is highly dependent on the degree of the stretch ACKs caused by receiver or network ACK compression mechanisms (e.g., delayed-ACK, GRO, etc). But slow start algorithm is to send twice the amount of packets of packets left so it should process a stretch ACK of degree N as if N ACKs of degree 1, then exits when cwnd exceeds ssthresh. A follow up patch will use the remainder of the N (if greater than 1) to adjust cwnd in the congestion avoidance phase. In addition this patch retires the experimental limited slow start (LSS) feature. LSS has multiple drawbacks but questionable benefit. The fractional cwnd increase in LSS requires a loop in slow start even though it's rarely used. Configuring such an increase step via a global sysctl on different BDPS seems hard. Finally and most importantly the slow start overshoot concern is now better covered by the Hybrid slow start (hystart) enabled by default. Signed-off-by: Yuchung Cheng <ycheng@google.com> Signed-off-by: Neal Cardwell <ncardwell@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
243 lines
6.2 KiB
C
243 lines
6.2 KiB
C
/*
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* Binary Increase Congestion control for TCP
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* Home page:
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* http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
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* This is from the implementation of BICTCP in
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* Lison-Xu, Kahaled Harfoush, and Injong Rhee.
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* "Binary Increase Congestion Control for Fast, Long Distance
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* Networks" in InfoComm 2004
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* Available from:
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* http://netsrv.csc.ncsu.edu/export/bitcp.pdf
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*
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* Unless BIC is enabled and congestion window is large
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* this behaves the same as the original Reno.
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <net/tcp.h>
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#define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
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* max_cwnd = snd_cwnd * beta
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*/
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#define BICTCP_B 4 /*
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* In binary search,
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* go to point (max+min)/N
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*/
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static int fast_convergence = 1;
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static int max_increment = 16;
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static int low_window = 14;
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static int beta = 819; /* = 819/1024 (BICTCP_BETA_SCALE) */
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static int initial_ssthresh;
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static int smooth_part = 20;
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module_param(fast_convergence, int, 0644);
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MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
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module_param(max_increment, int, 0644);
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MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
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module_param(low_window, int, 0644);
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MODULE_PARM_DESC(low_window, "lower bound on congestion window (for TCP friendliness)");
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module_param(beta, int, 0644);
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MODULE_PARM_DESC(beta, "beta for multiplicative increase");
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module_param(initial_ssthresh, int, 0644);
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MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
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module_param(smooth_part, int, 0644);
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MODULE_PARM_DESC(smooth_part, "log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax");
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/* BIC TCP Parameters */
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struct bictcp {
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u32 cnt; /* increase cwnd by 1 after ACKs */
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u32 last_max_cwnd; /* last maximum snd_cwnd */
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u32 loss_cwnd; /* congestion window at last loss */
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u32 last_cwnd; /* the last snd_cwnd */
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u32 last_time; /* time when updated last_cwnd */
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u32 epoch_start; /* beginning of an epoch */
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#define ACK_RATIO_SHIFT 4
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u32 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */
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};
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static inline void bictcp_reset(struct bictcp *ca)
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{
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ca->cnt = 0;
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ca->last_max_cwnd = 0;
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ca->last_cwnd = 0;
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ca->last_time = 0;
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ca->epoch_start = 0;
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ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
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}
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static void bictcp_init(struct sock *sk)
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{
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struct bictcp *ca = inet_csk_ca(sk);
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bictcp_reset(ca);
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ca->loss_cwnd = 0;
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if (initial_ssthresh)
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tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
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}
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/*
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* Compute congestion window to use.
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*/
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static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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{
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if (ca->last_cwnd == cwnd &&
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(s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
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return;
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ca->last_cwnd = cwnd;
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ca->last_time = tcp_time_stamp;
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if (ca->epoch_start == 0) /* record the beginning of an epoch */
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ca->epoch_start = tcp_time_stamp;
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/* start off normal */
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if (cwnd <= low_window) {
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ca->cnt = cwnd;
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return;
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}
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/* binary increase */
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if (cwnd < ca->last_max_cwnd) {
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__u32 dist = (ca->last_max_cwnd - cwnd)
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/ BICTCP_B;
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if (dist > max_increment)
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/* linear increase */
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ca->cnt = cwnd / max_increment;
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else if (dist <= 1U)
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/* binary search increase */
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ca->cnt = (cwnd * smooth_part) / BICTCP_B;
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else
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/* binary search increase */
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ca->cnt = cwnd / dist;
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} else {
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/* slow start AMD linear increase */
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if (cwnd < ca->last_max_cwnd + BICTCP_B)
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/* slow start */
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ca->cnt = (cwnd * smooth_part) / BICTCP_B;
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else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
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/* slow start */
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ca->cnt = (cwnd * (BICTCP_B-1))
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/ (cwnd - ca->last_max_cwnd);
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else
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/* linear increase */
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ca->cnt = cwnd / max_increment;
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}
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/* if in slow start or link utilization is very low */
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if (ca->last_max_cwnd == 0) {
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if (ca->cnt > 20) /* increase cwnd 5% per RTT */
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ca->cnt = 20;
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}
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ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
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if (ca->cnt == 0) /* cannot be zero */
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ca->cnt = 1;
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}
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static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked,
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u32 in_flight)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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if (!tcp_is_cwnd_limited(sk, in_flight))
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return;
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if (tp->snd_cwnd <= tp->snd_ssthresh)
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tcp_slow_start(tp, acked);
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else {
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bictcp_update(ca, tp->snd_cwnd);
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tcp_cong_avoid_ai(tp, ca->cnt);
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}
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}
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/*
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* behave like Reno until low_window is reached,
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* then increase congestion window slowly
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*/
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static u32 bictcp_recalc_ssthresh(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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ca->epoch_start = 0; /* end of epoch */
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/* Wmax and fast convergence */
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if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
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ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
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/ (2 * BICTCP_BETA_SCALE);
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else
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ca->last_max_cwnd = tp->snd_cwnd;
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ca->loss_cwnd = tp->snd_cwnd;
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if (tp->snd_cwnd <= low_window)
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return max(tp->snd_cwnd >> 1U, 2U);
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else
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return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
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}
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static u32 bictcp_undo_cwnd(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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const struct bictcp *ca = inet_csk_ca(sk);
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return max(tp->snd_cwnd, ca->loss_cwnd);
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}
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static void bictcp_state(struct sock *sk, u8 new_state)
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{
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if (new_state == TCP_CA_Loss)
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bictcp_reset(inet_csk_ca(sk));
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}
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/* Track delayed acknowledgment ratio using sliding window
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* ratio = (15*ratio + sample) / 16
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*/
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static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt)
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{
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const struct inet_connection_sock *icsk = inet_csk(sk);
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if (icsk->icsk_ca_state == TCP_CA_Open) {
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struct bictcp *ca = inet_csk_ca(sk);
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cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
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ca->delayed_ack += cnt;
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}
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}
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static struct tcp_congestion_ops bictcp __read_mostly = {
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.init = bictcp_init,
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.ssthresh = bictcp_recalc_ssthresh,
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.cong_avoid = bictcp_cong_avoid,
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.set_state = bictcp_state,
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.undo_cwnd = bictcp_undo_cwnd,
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.pkts_acked = bictcp_acked,
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.owner = THIS_MODULE,
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.name = "bic",
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};
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static int __init bictcp_register(void)
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{
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BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
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return tcp_register_congestion_control(&bictcp);
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}
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static void __exit bictcp_unregister(void)
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{
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tcp_unregister_congestion_control(&bictcp);
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}
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module_init(bictcp_register);
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module_exit(bictcp_unregister);
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MODULE_AUTHOR("Stephen Hemminger");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("BIC TCP");
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