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 48 #define DCTCP_MAX_ALPHA 1024U 49 50 struct dctcp { 51 u32 acked_bytes_ecn; 52 u32 acked_bytes_total; 53 u32 prior_snd_una; 54 u32 prior_rcv_nxt; 55 u32 dctcp_alpha; 56 u32 next_seq; 57 u32 ce_state; 58 u32 loss_cwnd; 59 }; 60 61 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */ 62 module_param(dctcp_shift_g, uint, 0644); 63 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha"); 64 65 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA; 66 module_param(dctcp_alpha_on_init, uint, 0644); 67 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value"); 68 69 static unsigned int dctcp_clamp_alpha_on_loss __read_mostly; 70 module_param(dctcp_clamp_alpha_on_loss, uint, 0644); 71 MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss, 72 "parameter for clamping alpha on loss"); 73 74 static struct tcp_congestion_ops dctcp_reno; 75 76 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca) 77 { 78 ca->next_seq = tp->snd_nxt; 79 80 ca->acked_bytes_ecn = 0; 81 ca->acked_bytes_total = 0; 82 } 83 84 static void dctcp_init(struct sock *sk) 85 { 86 const struct tcp_sock *tp = tcp_sk(sk); 87 88 if ((tp->ecn_flags & TCP_ECN_OK) || 89 (sk->sk_state == TCP_LISTEN || 90 sk->sk_state == TCP_CLOSE)) { 91 struct dctcp *ca = inet_csk_ca(sk); 92 93 ca->prior_snd_una = tp->snd_una; 94 ca->prior_rcv_nxt = tp->rcv_nxt; 95 96 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); 97 98 ca->loss_cwnd = 0; 99 ca->ce_state = 0; 100 101 dctcp_reset(tp, ca); 102 return; 103 } 104 105 /* No ECN support? Fall back to Reno. Also need to clear 106 * ECT from sk since it is set during 3WHS for DCTCP. 107 */ 108 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 109 INET_ECN_dontxmit(sk); 110 } 111 112 static u32 dctcp_ssthresh(struct sock *sk) 113 { 114 struct dctcp *ca = inet_csk_ca(sk); 115 struct tcp_sock *tp = tcp_sk(sk); 116 117 ca->loss_cwnd = tp->snd_cwnd; 118 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 119 } 120 121 /* Minimal DCTP CE state machine: 122 * 123 * S: 0 <- last pkt was non-CE 124 * 1 <- last pkt was CE 125 */ 126 127 static void dctcp_ce_state_0_to_1(struct sock *sk) 128 { 129 struct dctcp *ca = inet_csk_ca(sk); 130 struct tcp_sock *tp = tcp_sk(sk); 131 132 if (!ca->ce_state) { 133 /* State has changed from CE=0 to CE=1, force an immediate 134 * ACK to reflect the new CE state. If an ACK was delayed, 135 * send that first to reflect the prior CE state. 136 */ 137 if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) 138 __tcp_send_ack(sk, ca->prior_rcv_nxt); 139 tcp_enter_quickack_mode(sk, 1); 140 } 141 142 ca->prior_rcv_nxt = tp->rcv_nxt; 143 ca->ce_state = 1; 144 145 tp->ecn_flags |= TCP_ECN_DEMAND_CWR; 146 } 147 148 static void dctcp_ce_state_1_to_0(struct sock *sk) 149 { 150 struct dctcp *ca = inet_csk_ca(sk); 151 struct tcp_sock *tp = tcp_sk(sk); 152 153 if (ca->ce_state) { 154 /* State has changed from CE=1 to CE=0, force an immediate 155 * ACK to reflect the new CE state. If an ACK was delayed, 156 * send that first to reflect the prior CE state. 157 */ 158 if (inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) 159 __tcp_send_ack(sk, ca->prior_rcv_nxt); 160 tcp_enter_quickack_mode(sk, 1); 161 } 162 163 ca->prior_rcv_nxt = tp->rcv_nxt; 164 ca->ce_state = 0; 165 166 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; 167 } 168 169 static void dctcp_update_alpha(struct sock *sk, u32 flags) 170 { 171 const struct tcp_sock *tp = tcp_sk(sk); 172 struct dctcp *ca = inet_csk_ca(sk); 173 u32 acked_bytes = tp->snd_una - ca->prior_snd_una; 174 175 /* If ack did not advance snd_una, count dupack as MSS size. 176 * If ack did update window, do not count it at all. 177 */ 178 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE)) 179 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss; 180 if (acked_bytes) { 181 ca->acked_bytes_total += acked_bytes; 182 ca->prior_snd_una = tp->snd_una; 183 184 if (flags & CA_ACK_ECE) 185 ca->acked_bytes_ecn += acked_bytes; 186 } 187 188 /* Expired RTT */ 189 if (!before(tp->snd_una, ca->next_seq)) { 190 u64 bytes_ecn = ca->acked_bytes_ecn; 191 u32 alpha = ca->dctcp_alpha; 192 193 /* alpha = (1 - g) * alpha + g * F */ 194 195 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 196 if (bytes_ecn) { 197 /* If dctcp_shift_g == 1, a 32bit value would overflow 198 * after 8 Mbytes. 199 */ 200 bytes_ecn <<= (10 - dctcp_shift_g); 201 do_div(bytes_ecn, max(1U, ca->acked_bytes_total)); 202 203 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA); 204 } 205 /* dctcp_alpha can be read from dctcp_get_info() without 206 * synchro, so we ask compiler to not use dctcp_alpha 207 * as a temporary variable in prior operations. 208 */ 209 WRITE_ONCE(ca->dctcp_alpha, alpha); 210 dctcp_reset(tp, ca); 211 } 212 } 213 214 static void dctcp_state(struct sock *sk, u8 new_state) 215 { 216 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) { 217 struct dctcp *ca = inet_csk_ca(sk); 218 219 /* If this extension is enabled, we clamp dctcp_alpha to 220 * max on packet loss; the motivation is that dctcp_alpha 221 * is an indicator to the extend of congestion and packet 222 * loss is an indicator of extreme congestion; setting 223 * this in practice turned out to be beneficial, and 224 * effectively assumes total congestion which reduces the 225 * window by half. 226 */ 227 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 228 } 229 } 230 231 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 232 { 233 switch (ev) { 234 case CA_EVENT_ECN_IS_CE: 235 dctcp_ce_state_0_to_1(sk); 236 break; 237 case CA_EVENT_ECN_NO_CE: 238 dctcp_ce_state_1_to_0(sk); 239 break; 240 default: 241 /* Don't care for the rest. */ 242 break; 243 } 244 } 245 246 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 247 union tcp_cc_info *info) 248 { 249 const struct dctcp *ca = inet_csk_ca(sk); 250 251 /* Fill it also in case of VEGASINFO due to req struct limits. 252 * We can still correctly retrieve it later. 253 */ 254 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 255 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 256 memset(&info->dctcp, 0, sizeof(info->dctcp)); 257 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 258 info->dctcp.dctcp_enabled = 1; 259 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 260 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 261 info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn; 262 info->dctcp.dctcp_ab_tot = ca->acked_bytes_total; 263 } 264 265 *attr = INET_DIAG_DCTCPINFO; 266 return sizeof(info->dctcp); 267 } 268 return 0; 269 } 270 271 static u32 dctcp_cwnd_undo(struct sock *sk) 272 { 273 const struct dctcp *ca = inet_csk_ca(sk); 274 275 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd); 276 } 277 278 static struct tcp_congestion_ops dctcp __read_mostly = { 279 .init = dctcp_init, 280 .in_ack_event = dctcp_update_alpha, 281 .cwnd_event = dctcp_cwnd_event, 282 .ssthresh = dctcp_ssthresh, 283 .cong_avoid = tcp_reno_cong_avoid, 284 .undo_cwnd = dctcp_cwnd_undo, 285 .set_state = dctcp_state, 286 .get_info = dctcp_get_info, 287 .flags = TCP_CONG_NEEDS_ECN, 288 .owner = THIS_MODULE, 289 .name = "dctcp", 290 }; 291 292 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 293 .ssthresh = tcp_reno_ssthresh, 294 .cong_avoid = tcp_reno_cong_avoid, 295 .undo_cwnd = tcp_reno_undo_cwnd, 296 .get_info = dctcp_get_info, 297 .owner = THIS_MODULE, 298 .name = "dctcp-reno", 299 }; 300 301 static int __init dctcp_register(void) 302 { 303 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 304 return tcp_register_congestion_control(&dctcp); 305 } 306 307 static void __exit dctcp_unregister(void) 308 { 309 tcp_unregister_congestion_control(&dctcp); 310 } 311 312 module_init(dctcp_register); 313 module_exit(dctcp_unregister); 314 315 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 316 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 317 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 318 319 MODULE_LICENSE("GPL v2"); 320 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 321