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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>
317 lines
7.4 KiB
C
317 lines
7.4 KiB
C
// SPDX-License-Identifier: GPL-2.0-only
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/*
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* H-TCP congestion control. The algorithm is detailed in:
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* R.N.Shorten, D.J.Leith:
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* "H-TCP: TCP for high-speed and long-distance networks"
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* Proc. PFLDnet, Argonne, 2004.
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* https://www.hamilton.ie/net/htcp3.pdf
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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 ALPHA_BASE (1<<7) /* 1.0 with shift << 7 */
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#define BETA_MIN (1<<6) /* 0.5 with shift << 7 */
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#define BETA_MAX 102 /* 0.8 with shift << 7 */
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static int use_rtt_scaling __read_mostly = 1;
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module_param(use_rtt_scaling, int, 0644);
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MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
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static int use_bandwidth_switch __read_mostly = 1;
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module_param(use_bandwidth_switch, int, 0644);
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MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
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struct htcp {
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u32 alpha; /* Fixed point arith, << 7 */
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u8 beta; /* Fixed point arith, << 7 */
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u8 modeswitch; /* Delay modeswitch
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until we had at least one congestion event */
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u16 pkts_acked;
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u32 packetcount;
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u32 minRTT;
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u32 maxRTT;
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u32 last_cong; /* Time since last congestion event end */
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u32 undo_last_cong;
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u32 undo_maxRTT;
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u32 undo_old_maxB;
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/* Bandwidth estimation */
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u32 minB;
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u32 maxB;
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u32 old_maxB;
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u32 Bi;
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u32 lasttime;
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};
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static inline u32 htcp_cong_time(const struct htcp *ca)
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{
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return jiffies - ca->last_cong;
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}
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static inline u32 htcp_ccount(const struct htcp *ca)
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{
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return htcp_cong_time(ca) / ca->minRTT;
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}
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static inline void htcp_reset(struct htcp *ca)
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{
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ca->undo_last_cong = ca->last_cong;
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ca->undo_maxRTT = ca->maxRTT;
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ca->undo_old_maxB = ca->old_maxB;
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ca->last_cong = jiffies;
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}
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static u32 htcp_cwnd_undo(struct sock *sk)
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{
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struct htcp *ca = inet_csk_ca(sk);
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if (ca->undo_last_cong) {
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ca->last_cong = ca->undo_last_cong;
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ca->maxRTT = ca->undo_maxRTT;
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ca->old_maxB = ca->undo_old_maxB;
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ca->undo_last_cong = 0;
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}
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return tcp_reno_undo_cwnd(sk);
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}
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static inline void measure_rtt(struct sock *sk, u32 srtt)
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{
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const struct inet_connection_sock *icsk = inet_csk(sk);
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struct htcp *ca = inet_csk_ca(sk);
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/* keep track of minimum RTT seen so far, minRTT is zero at first */
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if (ca->minRTT > srtt || !ca->minRTT)
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ca->minRTT = srtt;
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/* max RTT */
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if (icsk->icsk_ca_state == TCP_CA_Open) {
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if (ca->maxRTT < ca->minRTT)
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ca->maxRTT = ca->minRTT;
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if (ca->maxRTT < srtt &&
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srtt <= ca->maxRTT + msecs_to_jiffies(20))
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ca->maxRTT = srtt;
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}
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}
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static void measure_achieved_throughput(struct sock *sk,
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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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const struct tcp_sock *tp = tcp_sk(sk);
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struct htcp *ca = inet_csk_ca(sk);
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u32 now = tcp_jiffies32;
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if (icsk->icsk_ca_state == TCP_CA_Open)
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ca->pkts_acked = sample->pkts_acked;
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if (sample->rtt_us > 0)
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measure_rtt(sk, usecs_to_jiffies(sample->rtt_us));
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if (!use_bandwidth_switch)
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return;
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/* achieved throughput calculations */
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if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_CA_Disorder))) {
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ca->packetcount = 0;
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ca->lasttime = now;
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return;
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}
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ca->packetcount += sample->pkts_acked;
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if (ca->packetcount >= tcp_snd_cwnd(tp) - (ca->alpha >> 7 ? : 1) &&
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now - ca->lasttime >= ca->minRTT &&
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ca->minRTT > 0) {
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__u32 cur_Bi = ca->packetcount * HZ / (now - ca->lasttime);
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if (htcp_ccount(ca) <= 3) {
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/* just after backoff */
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ca->minB = ca->maxB = ca->Bi = cur_Bi;
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} else {
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ca->Bi = (3 * ca->Bi + cur_Bi) / 4;
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if (ca->Bi > ca->maxB)
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ca->maxB = ca->Bi;
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if (ca->minB > ca->maxB)
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ca->minB = ca->maxB;
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}
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ca->packetcount = 0;
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ca->lasttime = now;
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}
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}
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static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
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{
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if (use_bandwidth_switch) {
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u32 maxB = ca->maxB;
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u32 old_maxB = ca->old_maxB;
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ca->old_maxB = ca->maxB;
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if (!between(5 * maxB, 4 * old_maxB, 6 * old_maxB)) {
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ca->beta = BETA_MIN;
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ca->modeswitch = 0;
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return;
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}
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}
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if (ca->modeswitch && minRTT > msecs_to_jiffies(10) && maxRTT) {
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ca->beta = (minRTT << 7) / maxRTT;
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if (ca->beta < BETA_MIN)
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ca->beta = BETA_MIN;
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else if (ca->beta > BETA_MAX)
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ca->beta = BETA_MAX;
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} else {
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ca->beta = BETA_MIN;
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ca->modeswitch = 1;
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}
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}
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static inline void htcp_alpha_update(struct htcp *ca)
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{
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u32 minRTT = ca->minRTT;
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u32 factor = 1;
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u32 diff = htcp_cong_time(ca);
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if (diff > HZ) {
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diff -= HZ;
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factor = 1 + (10 * diff + ((diff / 2) * (diff / 2) / HZ)) / HZ;
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}
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if (use_rtt_scaling && minRTT) {
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u32 scale = (HZ << 3) / (10 * minRTT);
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/* clamping ratio to interval [0.5,10]<<3 */
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scale = min(max(scale, 1U << 2), 10U << 3);
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factor = (factor << 3) / scale;
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if (!factor)
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factor = 1;
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}
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ca->alpha = 2 * factor * ((1 << 7) - ca->beta);
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if (!ca->alpha)
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ca->alpha = ALPHA_BASE;
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}
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/*
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* After we have the rtt data to calculate beta, we'd still prefer to wait one
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* rtt before we adjust our beta to ensure we are working from a consistent
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* data.
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*
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* This function should be called when we hit a congestion event since only at
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* that point do we really have a real sense of maxRTT (the queues en route
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* were getting just too full now).
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*/
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static void htcp_param_update(struct sock *sk)
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{
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struct htcp *ca = inet_csk_ca(sk);
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u32 minRTT = ca->minRTT;
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u32 maxRTT = ca->maxRTT;
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htcp_beta_update(ca, minRTT, maxRTT);
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htcp_alpha_update(ca);
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/* add slowly fading memory for maxRTT to accommodate routing changes */
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if (minRTT > 0 && maxRTT > minRTT)
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ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
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}
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static u32 htcp_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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const struct htcp *ca = inet_csk_ca(sk);
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htcp_param_update(sk);
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return max((tcp_snd_cwnd(tp) * ca->beta) >> 7, 2U);
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}
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static void htcp_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 htcp *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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tcp_slow_start(tp, acked);
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else {
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/* In dangerous area, increase slowly.
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* In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
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*/
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if ((tp->snd_cwnd_cnt * ca->alpha)>>7 >= tcp_snd_cwnd(tp)) {
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if (tcp_snd_cwnd(tp) < tp->snd_cwnd_clamp)
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tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
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tp->snd_cwnd_cnt = 0;
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htcp_alpha_update(ca);
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} else
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tp->snd_cwnd_cnt += ca->pkts_acked;
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ca->pkts_acked = 1;
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}
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}
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static void htcp_init(struct sock *sk)
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{
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struct htcp *ca = inet_csk_ca(sk);
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memset(ca, 0, sizeof(struct htcp));
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ca->alpha = ALPHA_BASE;
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ca->beta = BETA_MIN;
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ca->pkts_acked = 1;
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ca->last_cong = jiffies;
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}
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static void htcp_state(struct sock *sk, u8 new_state)
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{
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switch (new_state) {
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case TCP_CA_Open:
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{
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struct htcp *ca = inet_csk_ca(sk);
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if (ca->undo_last_cong) {
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ca->last_cong = jiffies;
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ca->undo_last_cong = 0;
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}
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}
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break;
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case TCP_CA_CWR:
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case TCP_CA_Recovery:
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case TCP_CA_Loss:
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htcp_reset(inet_csk_ca(sk));
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break;
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}
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}
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static struct tcp_congestion_ops htcp __read_mostly = {
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.init = htcp_init,
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.ssthresh = htcp_recalc_ssthresh,
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.cong_avoid = htcp_cong_avoid,
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.set_state = htcp_state,
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.undo_cwnd = htcp_cwnd_undo,
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.pkts_acked = measure_achieved_throughput,
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.owner = THIS_MODULE,
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.name = "htcp",
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};
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static int __init htcp_register(void)
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{
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BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
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BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
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return tcp_register_congestion_control(&htcp);
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}
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static void __exit htcp_unregister(void)
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{
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tcp_unregister_congestion_control(&htcp);
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}
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module_init(htcp_register);
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module_exit(htcp_unregister);
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MODULE_AUTHOR("Baruch Even");
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MODULE_LICENSE("GPL");
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MODULE_DESCRIPTION("H-TCP");
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