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![Eric Dumazet](/assets/img/avatar_default.png)
We had various bugs over the years with code
breaking the assumption that tp->snd_cwnd is greater
than zero.
Lately, syzbot reported the WARN_ON_ONCE(!tp->prior_cwnd) added
in commit 8b8a321ff7
("tcp: fix zero cwnd in tcp_cwnd_reduction")
can trigger, and without a repro we would have to spend
considerable time finding the bug.
Instead of complaining too late, we want to catch where
and when tp->snd_cwnd is set to an illegal value.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Link: https://lore.kernel.org/r/20220405233538.947344-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
229 lines
6 KiB
C
229 lines
6 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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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 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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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_jiffies32 - 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_jiffies32;
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if (ca->epoch_start == 0) /* record the beginning of an epoch */
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ca->epoch_start = tcp_jiffies32;
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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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{
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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))
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return;
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if (tcp_in_slow_start(tp)) {
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acked = tcp_slow_start(tp, acked);
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if (!acked)
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return;
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}
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bictcp_update(ca, tcp_snd_cwnd(tp));
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tcp_cong_avoid_ai(tp, ca->cnt, acked);
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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 (tcp_snd_cwnd(tp) < ca->last_max_cwnd && fast_convergence)
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ca->last_max_cwnd = (tcp_snd_cwnd(tp) * (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 = tcp_snd_cwnd(tp);
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if (tcp_snd_cwnd(tp) <= low_window)
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return max(tcp_snd_cwnd(tp) >> 1U, 2U);
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else
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return max((tcp_snd_cwnd(tp) * beta) / BICTCP_BETA_SCALE, 2U);
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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, const struct ack_sample *sample)
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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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ca->delayed_ack += sample->pkts_acked -
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(ca->delayed_ack >> ACK_RATIO_SHIFT);
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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 = tcp_reno_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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