/* 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;
  	u32 loss_cwnd;
  };
  
  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->loss_cwnd = 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)
  {
  	struct dctcp *ca = inet_csk_ca(sk);
  	struct tcp_sock *tp = tcp_sk(sk);
  
  	ca->loss_cwnd = tp->snd_cwnd;
  	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 u32 dctcp_cwnd_undo(struct sock *sk)
  {
  	const struct dctcp *ca = inet_csk_ca(sk);
  
  	return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
  }
  
  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,
  	.undo_cwnd	= dctcp_cwnd_undo,
  	.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)");