/docs/aoa/

// SPDX-License-Identifier: GPL-2.0-only
/*
 * 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))