tcp_wdctcp.c

/* 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>

#include "wdctcp.h"

static struct tcp_congestion_ops wdctcp_reno;

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

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

static void tcp_wdctcp_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 tcp_wdctcp *ca = inet_csk_ca(sk);

		ca->obj = wdctcp_obj_create(sk);
		if (unlikely(!ca->obj))
			goto fallback;

		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->loss_cwnd = 0;
		ca->ce_state = 0;

		tcp_wdctcp_reset(tp, ca);
		return;
	}

fallback:
	/* No ECN support or wdctcp_obj_create failed?
	 * 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 = &wdctcp_reno;
	INET_ECN_dontxmit(sk);
}

static void tcp_wdctcp_release(struct sock *sk)
{
	const struct tcp_wdctcp *ca = inet_csk_ca(sk);

	wdctcp_obj_put(ca->obj);
}

static u32 tcp_wdctcp_ssthresh(struct sock *sk)
{
	struct tcp_wdctcp *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	u32 wnd = max(min(tp->snd_cwnd, tp->packets_out), 1U);
        ca->loss_cwnd = wnd;
        return max(wnd - ((wnd * ca->dctcp_alpha) >> 11U), 2U);
}

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

static void tcp_wdctcp_ce_state_0_to_1(struct sock *sk)
{
	struct tcp_wdctcp *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 tcp_wdctcp_ce_state_1_to_0(struct sock *sk)
{
	struct tcp_wdctcp *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 tcp_wdctcp_update_alpha(struct sock *sk, u32 flags)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct tcp_wdctcp *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)) {
		/* For avoiding denominator == 1. */
		if (ca->acked_bytes_total == 0)
			ca->acked_bytes_total = 1;

		/* alpha = (1 - g) * alpha + g * F */
		ca->dctcp_alpha = ca->dctcp_alpha -
				  (ca->dctcp_alpha >> dctcp_shift_g) +
				  (ca->acked_bytes_ecn << (10U - dctcp_shift_g)) /
				  ca->acked_bytes_total;

		if (ca->dctcp_alpha > DCTCP_MAX_ALPHA)
			/* Clamp dctcp_alpha to max. */
			ca->dctcp_alpha = DCTCP_MAX_ALPHA;

		tcp_wdctcp_reset(tp, ca);
	}
}

static void tcp_wdctcp_state(struct sock *sk, u8 new_state)
{
	if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
		struct tcp_wdctcp *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 tcp_wdctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
{
	struct tcp_wdctcp *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 tcp_wdctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
{
	switch (ev) {
	case CA_EVENT_ECN_IS_CE:
		tcp_wdctcp_ce_state_0_to_1(sk);
		break;
	case CA_EVENT_ECN_NO_CE:
		tcp_wdctcp_ce_state_1_to_0(sk);
		break;
	case CA_EVENT_DELAYED_ACK:
	case CA_EVENT_NON_DELAYED_ACK:
		tcp_wdctcp_update_ack_reserved(sk, ev);
		break;
	default:
		/* Don't care for the rest. */
		break;
	}
}

static void tcp_wdctcp_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
{
	const struct tcp_wdctcp *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))) {
		struct tcp_dctcp_info info;

		memset(&info, 0, sizeof(info));
		if (inet_csk(sk)->icsk_ca_ops != &wdctcp_reno) {
			info.dctcp_enabled = 1;
			info.dctcp_ce_state = (u16) ca->ce_state;
			info.dctcp_alpha = ca->dctcp_alpha;
			info.dctcp_ab_ecn = ca->acked_bytes_ecn;
			info.dctcp_ab_tot = ca->acked_bytes_total;
		}

		nla_put(skb, INET_DIAG_DCTCPINFO, sizeof(info), &info);
	}
}

/* In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd (or alternative w),
 * for every packet that was ACKed.
 */
static void tcp_wdctcp_cong_avoid_ai(struct sock *sk, u32 w, u32 acked)
{
	struct tcp_wdctcp *ca = inet_csk_ca(sk);
	struct tcp_sock *tp = tcp_sk(sk);

	/* If credits accumulated at a higher w, apply them gently now. */
	if (tp->snd_cwnd_cnt >= w) {
		tp->snd_cwnd_cnt = 0;
		tp->snd_cwnd++;
	}

	/* Weighted increase snd_cwnd_cnt instead of adding acked directly. */
	ca->weight_acked_cnt += ca->obj->weight * acked;
	if (ca->weight_acked_cnt >= wdctcp_precision) {
		u32 delta = ca->weight_acked_cnt / wdctcp_precision;

		ca->weight_acked_cnt -= delta * wdctcp_precision;
		tp->snd_cwnd_cnt += delta;
	}

	if (tp->snd_cwnd_cnt >= w) {
		u32 delta = tp->snd_cwnd_cnt / w;

		tp->snd_cwnd_cnt -= delta * w;
		tp->snd_cwnd += delta;
	}
	tp->snd_cwnd = min(tp->snd_cwnd, tp->snd_cwnd_clamp);
}

/*
 * Weighted DCTCP congestion control
 */
static void tcp_wdctcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (!tcp_is_cwnd_limited(sk))
		return;

	/* In "safe" area, increase. */
	if (tp->snd_cwnd <= tp->snd_ssthresh) {
		acked = tcp_slow_start(tp, acked);
		if (!acked)
			return;
	}
	/* In dangerous area, increase slowly. */
	tcp_wdctcp_cong_avoid_ai(sk, tp->snd_cwnd, acked);
}

static u32 tcp_wdctcp_undo_cwnd(struct sock *sk)
{
	const struct tcp_wdctcp *ca = inet_csk_ca(sk);

	return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
}

static struct tcp_congestion_ops tcp_wdctcp __read_mostly = {
	.init		= tcp_wdctcp_init,
	.release	= tcp_wdctcp_release,
	.in_ack_event   = tcp_wdctcp_update_alpha,
	.cwnd_event	= tcp_wdctcp_cwnd_event,
	.ssthresh	= tcp_wdctcp_ssthresh,
	.cong_avoid	= tcp_wdctcp_cong_avoid,
	.undo_cwnd	= tcp_wdctcp_undo_cwnd,
	.set_state	= tcp_wdctcp_state,
	.flags		= TCP_CONG_NEEDS_ECN,

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

static struct tcp_congestion_ops wdctcp_reno __read_mostly = {
	.ssthresh	= tcp_reno_ssthresh,
	.cong_avoid	= tcp_reno_cong_avoid,

	.owner		= THIS_MODULE,
	.name		= "wdctcp-reno",
};

int __init tcp_wdctcp_register(void)
{
	BUILD_BUG_ON(sizeof(struct tcp_wdctcp) > ICSK_CA_PRIV_SIZE);
	return tcp_register_congestion_control(&tcp_wdctcp);
}

void tcp_wdctcp_unregister(void)
{
	tcp_unregister_congestion_control(&tcp_wdctcp);
}