/* DataCenter TCP (DCTCP) congestion control.
 *
 * http://simula.stanford.edu/~alizade/Site/DCTCP.html
 *
 * This is an implementation of DCTCP over Reno, an enhancement to the
 * TCP congestion control algorithm designed for data centers. DCTCP
 * leverages Explicit Congestion Notification (ECN) in the network to
 * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
 * the following three data center transport requirements:
 *
 *  - High burst tolerance (incast due to partition/aggregate)
 *  - Low latency (short flows, queries)
 *  - High throughput (continuous data updates, large file transfers)
 *    with commodity shallow buffered switches
 *
 * The algorithm is described in detail in the following two papers:
 *
 * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
 *    Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
 *      "Data Center TCP (DCTCP)", Data Center Networks session
 *      Proc. ACM SIGCOMM, New Delhi, 2010.
 *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
 *
 * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
 *      "Analysis of DCTCP: Stability, Convergence, and Fairness"
 *      Proc. ACM SIGMETRICS, San Jose, 2011.
 *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
 *
 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
 *
 * Authors:
 *
 *      Daniel Borkmann <dborkman@redhat.com>
 *      Florian Westphal <fw@strlen.de>
 *      Glenn Judd <glenn.judd@morganstanley.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or (at
 * your option) any later version.
 */

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

#define DCTCP_MAX_ALPHA 1024U

struct dctcp {
        u32 acked_bytes_ecn;
        u32 acked_bytes_total;
        u32 prior_snd_una;
        u32 prior_rcv_nxt;
        u32 dctcp_alpha;
        u32 next_seq;
        u32 ce_state;
        u32 delayed_ack_reserved;
};

static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
module_param(dctcp_shift_g, uint, 0644);
MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");

static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
module_param(dctcp_alpha_on_init, uint, 0644);
MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");

static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
                 "parameter for clamping alpha on loss");

static struct tcp_congestion_ops dctcp_reno;

static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
{
        ca->next_seq = tp->snd_nxt;

        ca->acked_bytes_ecn = 0;
        ca->acked_bytes_total = 0;
}

static void dctcp_init(struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        if ((tp->ecn_flags & TCP_ECN_OK) ||
            (sk->sk_state == TCP_LISTEN ||
             sk->sk_state == TCP_CLOSE)) {
                struct dctcp *ca = inet_csk_ca(sk);

                ca->prior_snd_una = tp->snd_una;
                ca->prior_rcv_nxt = tp->rcv_nxt;

                ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);

                ca->delayed_ack_reserved = 0;
                ca->ce_state = 0;

                dctcp_reset(tp, ca);
                return;
        }

        /* No ECN support? Fall back to Reno. Also need to clear
         * ECT from sk since it is set during 3WHS for DCTCP.
         */
        inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
        INET_ECN_dontxmit(sk);
}

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

        return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
}

/* Minimal DCTP CE state machine:
 *
 * S:   0 <- last pkt was non-CE
 *      1 <- last pkt was CE
 */

static void dctcp_ce_state_0_to_1(struct sock *sk)
{
        struct dctcp *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);

        /* State has changed from CE=0 to CE=1 and delayed
         * ACK has not sent yet.
         */
        if (!ca->ce_state && ca->delayed_ack_reserved) {
                u32 tmp_rcv_nxt;

                /* Save current rcv_nxt. */
                tmp_rcv_nxt = tp->rcv_nxt;

                /* Generate previous ack with CE=0. */
                tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
                tp->rcv_nxt = ca->prior_rcv_nxt;

                tcp_send_ack(sk);

                /* Recover current rcv_nxt. */
                tp->rcv_nxt = tmp_rcv_nxt;
        }

        ca->prior_rcv_nxt = tp->rcv_nxt;
        ca->ce_state = 1;

        tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
}

static void dctcp_ce_state_1_to_0(struct sock *sk)
{
        struct dctcp *ca = inet_csk_ca(sk);
        struct tcp_sock *tp = tcp_sk(sk);

        /* State has changed from CE=1 to CE=0 and delayed
         * ACK has not sent yet.
         */
        if (ca->ce_state && ca->delayed_ack_reserved) {
                u32 tmp_rcv_nxt;

                /* Save current rcv_nxt. */
                tmp_rcv_nxt = tp->rcv_nxt;

                /* Generate previous ack with CE=1. */
                tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
                tp->rcv_nxt = ca->prior_rcv_nxt;

                tcp_send_ack(sk);

                /* Recover current rcv_nxt. */
                tp->rcv_nxt = tmp_rcv_nxt;
        }

        ca->prior_rcv_nxt = tp->rcv_nxt;
        ca->ce_state = 0;

        tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}

static void dctcp_update_alpha(struct sock *sk, u32 flags)
{
        const struct tcp_sock *tp = tcp_sk(sk);
        struct dctcp *ca = inet_csk_ca(sk);
        u32 acked_bytes = tp->snd_una - ca->prior_snd_una;

        /* If ack did not advance snd_una, count dupack as MSS size.
         * If ack did update window, do not count it at all.
         */
        if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
                acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
        if (acked_bytes) {
                ca->acked_bytes_total += acked_bytes;
                ca->prior_snd_una = tp->snd_una;

                if (flags & CA_ACK_ECE)
                        ca->acked_bytes_ecn += acked_bytes;
        }

        /* Expired RTT */
        if (!before(tp->snd_una, ca->next_seq)) {
                u64 bytes_ecn = ca->acked_bytes_ecn;
                u32 alpha = ca->dctcp_alpha;

                /* alpha = (1 - g) * alpha + g * F */

                alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
                if (bytes_ecn) {
                        /* If dctcp_shift_g == 1, a 32bit value would overflow
                         * after 8 Mbytes.
                         */
                        bytes_ecn <<= (10 - dctcp_shift_g);
                        do_div(bytes_ecn, max(1U, ca->acked_bytes_total));

                        alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
                }
                /* dctcp_alpha can be read from dctcp_get_info() without
                 * synchro, so we ask compiler to not use dctcp_alpha
                 * as a temporary variable in prior operations.
                 */
                WRITE_ONCE(ca->dctcp_alpha, alpha);
                dctcp_reset(tp, ca);
        }
}

static void dctcp_state(struct sock *sk, u8 new_state)
{
        if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
                struct dctcp *ca = inet_csk_ca(sk);

                /* If this extension is enabled, we clamp dctcp_alpha to
                 * max on packet loss; the motivation is that dctcp_alpha
                 * is an indicator to the extend of congestion and packet
                 * loss is an indicator of extreme congestion; setting
                 * this in practice turned out to be beneficial, and
                 * effectively assumes total congestion which reduces the
                 * window by half.
                 */
                ca->dctcp_alpha = DCTCP_MAX_ALPHA;
        }
}

static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
{
        struct dctcp *ca = inet_csk_ca(sk);

        switch (ev) {
        case CA_EVENT_DELAYED_ACK:
                if (!ca->delayed_ack_reserved)
                        ca->delayed_ack_reserved = 1;
                break;
        case CA_EVENT_NON_DELAYED_ACK:
                if (ca->delayed_ack_reserved)
                        ca->delayed_ack_reserved = 0;
                break;
        default:
                /* Don't care for the rest. */
                break;
        }
}

static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
{
        switch (ev) {
        case CA_EVENT_ECN_IS_CE:
                dctcp_ce_state_0_to_1(sk);
                break;
        case CA_EVENT_ECN_NO_CE:
                dctcp_ce_state_1_to_0(sk);
                break;
        case CA_EVENT_DELAYED_ACK:
        case CA_EVENT_NON_DELAYED_ACK:
                dctcp_update_ack_reserved(sk, ev);
                break;
        default:
                /* Don't care for the rest. */
                break;
        }
}

static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
                             union tcp_cc_info *info)
{
        const struct dctcp *ca = inet_csk_ca(sk);

        /* Fill it also in case of VEGASINFO due to req struct limits.
         * We can still correctly retrieve it later.
         */
        if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
            ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
                memset(info, 0, sizeof(struct tcp_dctcp_info));
                if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
                        info->dctcp.dctcp_enabled = 1;
                        info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
                        info->dctcp.dctcp_alpha = ca->dctcp_alpha;
                        info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
                        info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
                }

                *attr = INET_DIAG_DCTCPINFO;
                return sizeof(*info);
        }
        return 0;
}

static struct tcp_congestion_ops dctcp __read_mostly = {
        .init           = dctcp_init,
        .in_ack_event   = dctcp_update_alpha,
        .cwnd_event     = dctcp_cwnd_event,
        .ssthresh       = dctcp_ssthresh,
        .cong_avoid     = tcp_reno_cong_avoid,
        .set_state      = dctcp_state,
        .get_info       = dctcp_get_info,
        .flags          = TCP_CONG_NEEDS_ECN,
        .owner          = THIS_MODULE,
        .name           = "dctcp",
};

static struct tcp_congestion_ops dctcp_reno __read_mostly = {
        .ssthresh       = tcp_reno_ssthresh,
        .cong_avoid     = tcp_reno_cong_avoid,
        .get_info       = dctcp_get_info,
        .owner          = THIS_MODULE,
        .name           = "dctcp-reno",
};

static int __init dctcp_register(void)
{
        BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
        return tcp_register_congestion_control(&dctcp);
}

static void __exit dctcp_unregister(void)
{
        tcp_unregister_congestion_control(&dctcp);
}

module_init(dctcp_register);
module_exit(dctcp_unregister);

MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");

MODULE_LICENSE("GPL v2");
MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");