// SPDX-License-Identifier: GPL-2.0-only
/* Tom Kelly's Scalable TCP
 *
 * See http://www.deneholme.net/tom/scalable/
 *
 * John Heffner <jheffner@sc.edu>
 */

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

/* These factors derived from the recommended values in the aer:
 * .01 and 7/8.
 */
#define TCP_SCALABLE_AI_CNT     100U
#define TCP_SCALABLE_MD_SCALE   3

static void tcp_scalable_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
        struct tcp_sock *tp = tcp_sk(sk);

        if (!tcp_is_cwnd_limited(sk))
                return;

        if (tcp_in_slow_start(tp)) {
                acked = tcp_slow_start(tp, acked);
                if (!acked)
                        return;
        }
        tcp_cong_avoid_ai(tp, min(tcp_snd_cwnd(tp), TCP_SCALABLE_AI_CNT),
                          acked);
}

static u32 tcp_scalable_ssthresh(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        return max(tcp_snd_cwnd(tp) - (tcp_snd_cwnd(tp)>>TCP_SCALABLE_MD_SCALE), 2U);
}

static struct tcp_congestion_ops tcp_scalable __read_mostly = {
        .ssthresh       = tcp_scalable_ssthresh,
        .undo_cwnd      = tcp_reno_undo_cwnd,
        .cong_avoid     = tcp_scalable_cong_avoid,

        .owner          = THIS_MODULE,
        .name           = "scalable",
};

static int __init tcp_scalable_register(void)
{
        return tcp_register_congestion_control(&tcp_scalable);
}

static void __exit tcp_scalable_unregister(void)
{
        tcp_unregister_congestion_control(&tcp_scalable);
}

module_init(tcp_scalable_register);
module_exit(tcp_scalable_unregister);

MODULE_AUTHOR("John Heffner");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Scalable TCP");