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* H-TCP congestion control. The algorithm is detailed in:
* R.N.Shorten, D.J.Leith:
*   "H-TCP: TCP for high-speed and long-distance networks"
*   Proc. PFLDnet, Argonne, 2004.
* http://www.hamilton.ie/net/htcp3.pdf
*/

#include <linux/mm.h>
#include <linux/module.h>
#include <net/tcp.h>

#define ALPHA_BASE	(1<<7)	/* 1.0 with shift << 7 */
#define BETA_MIN	(1<<6)	/* 0.5 with shift << 7 */
#define BETA_MAX	102	/* 0.8 with shift << 7 */

static int use_rtt_scaling __read_mostly = 1;
module_param(use_rtt_scaling, int, 0644);
MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");

static int use_bandwidth_switch __read_mostly = 1;
module_param(use_bandwidth_switch, int, 0644);
MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");

struct htcp {
u32	alpha;		/* Fixed point arith, << 7 */
u8	beta;           /* Fixed point arith, << 7 */
u8	modeswitch;	/* Delay modeswitch
until we had at least one congestion event */
u16	pkts_acked;
u32	packetcount;
u32	minRTT;
u32	maxRTT;
u32	last_cong;	/* Time since last congestion event end */
u32	undo_last_cong;

u32	undo_maxRTT;
u32	undo_old_maxB;

/* Bandwidth estimation */
u32	minB;
u32	maxB;
u32	old_maxB;
u32	Bi;
u32	lasttime;
};

static inline u32 htcp_cong_time(const struct htcp *ca)
{
return jiffies - ca->last_cong;
}

static inline u32 htcp_ccount(const struct htcp *ca)
{
return htcp_cong_time(ca) / ca->minRTT;
}

static inline void htcp_reset(struct htcp *ca)
{
ca->undo_last_cong = ca->last_cong;
ca->undo_maxRTT = ca->maxRTT;
ca->undo_old_maxB = ca->old_maxB;

ca->last_cong = jiffies;
}

static u32 htcp_cwnd_undo(struct sock *sk)
{
struct htcp *ca = inet_csk_ca(sk);

if (ca->undo_last_cong) {
ca->last_cong = ca->undo_last_cong;
ca->maxRTT = ca->undo_maxRTT;
ca->old_maxB = ca->undo_old_maxB;
ca->undo_last_cong = 0;
}

return tcp_reno_undo_cwnd(sk);
}

static inline void measure_rtt(struct sock *sk, u32 srtt)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
struct htcp *ca = inet_csk_ca(sk);

/* keep track of minimum RTT seen so far, minRTT is zero at first */
if (ca->minRTT > srtt || !ca->minRTT)
ca->minRTT = srtt;

/* max RTT */
if (icsk->icsk_ca_state == TCP_CA_Open) {
if (ca->maxRTT < ca->minRTT)
ca->maxRTT = ca->minRTT;
if (ca->maxRTT < srtt &&
srtt <= ca->maxRTT + msecs_to_jiffies(20))
ca->maxRTT = srtt;
}
}

static void measure_achieved_throughput(struct sock *sk,
const struct ack_sample *sample)
{
const struct inet_connection_sock *icsk = inet_csk(sk);
const struct tcp_sock *tp = tcp_sk(sk);
struct htcp *ca = inet_csk_ca(sk);
u32 now = tcp_jiffies32;

if (icsk->icsk_ca_state == TCP_CA_Open)
ca->pkts_acked = sample->pkts_acked;

if (sample->rtt_us > 0)
measure_rtt(sk, usecs_to_jiffies(sample->rtt_us));

if (!use_bandwidth_switch)
return;

/* achieved throughput calculations */
if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_CA_Disorder))) {
ca->packetcount = 0;
ca->lasttime = now;
return;
}

ca->packetcount += sample->pkts_acked;

if (ca->packetcount >= tp->snd_cwnd - (ca->alpha >> 7 ? : 1) &&
now - ca->lasttime >= ca->minRTT &&
ca->minRTT > 0) {
__u32 cur_Bi = ca->packetcount * HZ / (now - ca->lasttime);

if (htcp_ccount(ca) <= 3) {
/* just after backoff */
ca->minB = ca->maxB = ca->Bi = cur_Bi;
} else {
ca->Bi = (3 * ca->Bi + cur_Bi) / 4;
if (ca->Bi > ca->maxB)
ca->maxB = ca->Bi;
if (ca->minB > ca->maxB)
ca->minB = ca->maxB;
}
ca->packetcount = 0;
ca->lasttime = now;
}
}

static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
{
if (use_bandwidth_switch) {
u32 maxB = ca->maxB;
u32 old_maxB = ca->old_maxB;

ca->old_maxB = ca->maxB;
if (!between(5 * maxB, 4 * old_maxB, 6 * old_maxB)) {
ca->beta = BETA_MIN;
ca->modeswitch = 0;
return;
}
}

if (ca->modeswitch && minRTT > msecs_to_jiffies(10) && maxRTT) {
ca->beta = (minRTT << 7) / maxRTT;
if (ca->beta < BETA_MIN)
ca->beta = BETA_MIN;
else if (ca->beta > BETA_MAX)
ca->beta = BETA_MAX;
} else {
ca->beta = BETA_MIN;
ca->modeswitch = 1;
}
}

static inline void htcp_alpha_update(struct htcp *ca)
{
u32 minRTT = ca->minRTT;
u32 factor = 1;
u32 diff = htcp_cong_time(ca);

if (diff > HZ) {
diff -= HZ;
factor = 1 + (10 * diff + ((diff / 2) * (diff / 2) / HZ)) / HZ;
}

if (use_rtt_scaling && minRTT) {
u32 scale = (HZ << 3) / (10 * minRTT);

/* clamping ratio to interval [0.5,10]<<3 */
scale = min(max(scale, 1U << 2), 10U << 3);
factor = (factor << 3) / scale;
if (!factor)
factor = 1;
}

ca->alpha = 2 * factor * ((1 << 7) - ca->beta);
if (!ca->alpha)
ca->alpha = ALPHA_BASE;
}

/*
* After we have the rtt data to calculate beta, we'd still prefer to wait one
* rtt before we adjust our beta to ensure we are working from a consistent
* data.
*
* This function should be called when we hit a congestion event since only at
* that point do we really have a real sense of maxRTT (the queues en route
* were getting just too full now).
*/
static void htcp_param_update(struct sock *sk)
{
struct htcp *ca = inet_csk_ca(sk);
u32 minRTT = ca->minRTT;
u32 maxRTT = ca->maxRTT;

htcp_beta_update(ca, minRTT, maxRTT);
htcp_alpha_update(ca);

/* add slowly fading memory for maxRTT to accommodate routing changes */
if (minRTT > 0 && maxRTT > minRTT)
ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
}

static u32 htcp_recalc_ssthresh(struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
const struct htcp *ca = inet_csk_ca(sk);

htcp_param_update(sk);
return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
}

static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
struct tcp_sock *tp = tcp_sk(sk);
struct htcp *ca = inet_csk_ca(sk);

if (!tcp_is_cwnd_limited(sk))
return;

if (tcp_in_slow_start(tp))
tcp_slow_start(tp, acked);
else {
/* In dangerous area, increase slowly.
* In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
*/
if ((tp->snd_cwnd_cnt * ca->alpha)>>7 >= tp->snd_cwnd) {
if (tp->snd_cwnd < tp->snd_cwnd_clamp)
tp->snd_cwnd++;
tp->snd_cwnd_cnt = 0;
htcp_alpha_update(ca);
} else
tp->snd_cwnd_cnt += ca->pkts_acked;

ca->pkts_acked = 1;
}
}

static void htcp_init(struct sock *sk)
{
struct htcp *ca = inet_csk_ca(sk);

memset(ca, 0, sizeof(struct htcp));
ca->alpha = ALPHA_BASE;
ca->beta = BETA_MIN;
ca->pkts_acked = 1;
ca->last_cong = jiffies;
}

static void htcp_state(struct sock *sk, u8 new_state)
{
switch (new_state) {
case TCP_CA_Open:
{
struct htcp *ca = inet_csk_ca(sk);

if (ca->undo_last_cong) {
ca->last_cong = jiffies;
ca->undo_last_cong = 0;
}
}
break;
case TCP_CA_CWR:
case TCP_CA_Recovery:
case TCP_CA_Loss:
htcp_reset(inet_csk_ca(sk));
break;
}
}

static struct tcp_congestion_ops htcp __read_mostly = {
.init		= htcp_init,
.ssthresh	= htcp_recalc_ssthresh,
.cong_avoid	= htcp_cong_avoid,
.set_state	= htcp_state,
.undo_cwnd	= htcp_cwnd_undo,
.pkts_acked	= measure_achieved_throughput,
.owner		= THIS_MODULE,
.name		= "htcp",
};

static int __init htcp_register(void)
{
BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
return tcp_register_congestion_control(&htcp);
}

static void __exit htcp_unregister(void)
{
tcp_unregister_congestion_control(&htcp);
}

module_init(htcp_register);
module_exit(htcp_unregister);

MODULE_AUTHOR("Baruch Even");