/* 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->dctcp, 0, sizeof(info->dctcp)); 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->dctcp); } 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, .undo_cwnd = tcp_reno_undo_cwnd, .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)");