1 /* DataCenter TCP (DCTCP) congestion control. 2 * 3 * http://simula.stanford.edu/~alizade/Site/DCTCP.html 4 * 5 * This is an implementation of DCTCP over Reno, an enhancement to the 6 * TCP congestion control algorithm designed for data centers. DCTCP 7 * leverages Explicit Congestion Notification (ECN) in the network to 8 * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet 9 * the following three data center transport requirements: 10 * 11 * - High burst tolerance (incast due to partition/aggregate) 12 * - Low latency (short flows, queries) 13 * - High throughput (continuous data updates, large file transfers) 14 * with commodity shallow buffered switches 15 * 16 * The algorithm is described in detail in the following two papers: 17 * 18 * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye, 19 * Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan: 20 * "Data Center TCP (DCTCP)", Data Center Networks session 21 * Proc. ACM SIGCOMM, New Delhi, 2010. 22 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf 23 * 24 * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar: 25 * "Analysis of DCTCP: Stability, Convergence, and Fairness" 26 * Proc. ACM SIGMETRICS, San Jose, 2011. 27 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf 28 * 29 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh. 30 * 31 * Authors: 32 * 33 * Daniel Borkmann <dborkman@redhat.com> 34 * Florian Westphal <fw@strlen.de> 35 * Glenn Judd <glenn.judd@morganstanley.com> 36 * 37 * This program is free software; you can redistribute it and/or modify 38 * it under the terms of the GNU General Public License as published by 39 * the Free Software Foundation; either version 2 of the License, or (at 40 * your option) any later version. 41 */ 42 43 #include <linux/module.h> 44 #include <linux/mm.h> 45 #include <net/tcp.h> 46 #include <linux/inet_diag.h> 47 #include "tcp_dctcp.h" 48 49 #define DCTCP_MAX_ALPHA 1024U 50 51 struct dctcp { 52 u32 acked_bytes_ecn; 53 u32 acked_bytes_total; 54 u32 prior_snd_una; 55 u32 prior_rcv_nxt; 56 u32 dctcp_alpha; 57 u32 next_seq; 58 u32 ce_state; 59 u32 loss_cwnd; 60 }; 61 62 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */ 63 module_param(dctcp_shift_g, uint, 0644); 64 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha"); 65 66 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA; 67 module_param(dctcp_alpha_on_init, uint, 0644); 68 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value"); 69 70 static struct tcp_congestion_ops dctcp_reno; 71 72 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca) 73 { 74 ca->next_seq = tp->snd_nxt; 75 76 ca->acked_bytes_ecn = 0; 77 ca->acked_bytes_total = 0; 78 } 79 80 static void dctcp_init(struct sock *sk) 81 { 82 const struct tcp_sock *tp = tcp_sk(sk); 83 84 if ((tp->ecn_flags & TCP_ECN_OK) || 85 (sk->sk_state == TCP_LISTEN || 86 sk->sk_state == TCP_CLOSE)) { 87 struct dctcp *ca = inet_csk_ca(sk); 88 89 ca->prior_snd_una = tp->snd_una; 90 ca->prior_rcv_nxt = tp->rcv_nxt; 91 92 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); 93 94 ca->loss_cwnd = 0; 95 ca->ce_state = 0; 96 97 dctcp_reset(tp, ca); 98 return; 99 } 100 101 /* No ECN support? Fall back to Reno. Also need to clear 102 * ECT from sk since it is set during 3WHS for DCTCP. 103 */ 104 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 105 INET_ECN_dontxmit(sk); 106 } 107 108 static u32 dctcp_ssthresh(struct sock *sk) 109 { 110 struct dctcp *ca = inet_csk_ca(sk); 111 struct tcp_sock *tp = tcp_sk(sk); 112 113 ca->loss_cwnd = tp->snd_cwnd; 114 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 115 } 116 117 static void dctcp_update_alpha(struct sock *sk, u32 flags) 118 { 119 const struct tcp_sock *tp = tcp_sk(sk); 120 struct dctcp *ca = inet_csk_ca(sk); 121 u32 acked_bytes = tp->snd_una - ca->prior_snd_una; 122 123 /* If ack did not advance snd_una, count dupack as MSS size. 124 * If ack did update window, do not count it at all. 125 */ 126 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE)) 127 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss; 128 if (acked_bytes) { 129 ca->acked_bytes_total += acked_bytes; 130 ca->prior_snd_una = tp->snd_una; 131 132 if (flags & CA_ACK_ECE) 133 ca->acked_bytes_ecn += acked_bytes; 134 } 135 136 /* Expired RTT */ 137 if (!before(tp->snd_una, ca->next_seq)) { 138 u64 bytes_ecn = ca->acked_bytes_ecn; 139 u32 alpha = ca->dctcp_alpha; 140 141 /* alpha = (1 - g) * alpha + g * F */ 142 143 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 144 if (bytes_ecn) { 145 /* If dctcp_shift_g == 1, a 32bit value would overflow 146 * after 8 Mbytes. 147 */ 148 bytes_ecn <<= (10 - dctcp_shift_g); 149 do_div(bytes_ecn, max(1U, ca->acked_bytes_total)); 150 151 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA); 152 } 153 /* dctcp_alpha can be read from dctcp_get_info() without 154 * synchro, so we ask compiler to not use dctcp_alpha 155 * as a temporary variable in prior operations. 156 */ 157 WRITE_ONCE(ca->dctcp_alpha, alpha); 158 dctcp_reset(tp, ca); 159 } 160 } 161 162 static void dctcp_react_to_loss(struct sock *sk) 163 { 164 struct dctcp *ca = inet_csk_ca(sk); 165 struct tcp_sock *tp = tcp_sk(sk); 166 167 ca->loss_cwnd = tp->snd_cwnd; 168 tp->snd_ssthresh = max(tp->snd_cwnd >> 1U, 2U); 169 } 170 171 static void dctcp_state(struct sock *sk, u8 new_state) 172 { 173 if (new_state == TCP_CA_Recovery && 174 new_state != inet_csk(sk)->icsk_ca_state) 175 dctcp_react_to_loss(sk); 176 /* We handle RTO in dctcp_cwnd_event to ensure that we perform only 177 * one loss-adjustment per RTT. 178 */ 179 } 180 181 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 182 { 183 struct dctcp *ca = inet_csk_ca(sk); 184 185 switch (ev) { 186 case CA_EVENT_ECN_IS_CE: 187 case CA_EVENT_ECN_NO_CE: 188 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state); 189 break; 190 case CA_EVENT_LOSS: 191 dctcp_react_to_loss(sk); 192 break; 193 default: 194 /* Don't care for the rest. */ 195 break; 196 } 197 } 198 199 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 200 union tcp_cc_info *info) 201 { 202 const struct dctcp *ca = inet_csk_ca(sk); 203 204 /* Fill it also in case of VEGASINFO due to req struct limits. 205 * We can still correctly retrieve it later. 206 */ 207 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 208 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 209 memset(&info->dctcp, 0, sizeof(info->dctcp)); 210 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 211 info->dctcp.dctcp_enabled = 1; 212 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 213 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 214 info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn; 215 info->dctcp.dctcp_ab_tot = ca->acked_bytes_total; 216 } 217 218 *attr = INET_DIAG_DCTCPINFO; 219 return sizeof(info->dctcp); 220 } 221 return 0; 222 } 223 224 static u32 dctcp_cwnd_undo(struct sock *sk) 225 { 226 const struct dctcp *ca = inet_csk_ca(sk); 227 228 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd); 229 } 230 231 static struct tcp_congestion_ops dctcp __read_mostly = { 232 .init = dctcp_init, 233 .in_ack_event = dctcp_update_alpha, 234 .cwnd_event = dctcp_cwnd_event, 235 .ssthresh = dctcp_ssthresh, 236 .cong_avoid = tcp_reno_cong_avoid, 237 .undo_cwnd = dctcp_cwnd_undo, 238 .set_state = dctcp_state, 239 .get_info = dctcp_get_info, 240 .flags = TCP_CONG_NEEDS_ECN, 241 .owner = THIS_MODULE, 242 .name = "dctcp", 243 }; 244 245 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 246 .ssthresh = tcp_reno_ssthresh, 247 .cong_avoid = tcp_reno_cong_avoid, 248 .undo_cwnd = tcp_reno_undo_cwnd, 249 .get_info = dctcp_get_info, 250 .owner = THIS_MODULE, 251 .name = "dctcp-reno", 252 }; 253 254 static int __init dctcp_register(void) 255 { 256 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 257 return tcp_register_congestion_control(&dctcp); 258 } 259 260 static void __exit dctcp_unregister(void) 261 { 262 tcp_unregister_congestion_control(&dctcp); 263 } 264 265 module_init(dctcp_register); 266 module_exit(dctcp_unregister); 267 268 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 269 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 270 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 271 272 MODULE_LICENSE("GPL v2"); 273 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 274