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 delayed_ack_reserved; 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 unsigned int dctcp_clamp_alpha_on_loss __read_mostly; 71 module_param(dctcp_clamp_alpha_on_loss, uint, 0644); 72 MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss, 73 "parameter for clamping alpha on loss"); 74 75 static struct tcp_congestion_ops dctcp_reno; 76 77 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca) 78 { 79 ca->next_seq = tp->snd_nxt; 80 81 ca->acked_bytes_ecn = 0; 82 ca->acked_bytes_total = 0; 83 } 84 85 static void dctcp_init(struct sock *sk) 86 { 87 const struct tcp_sock *tp = tcp_sk(sk); 88 89 if ((tp->ecn_flags & TCP_ECN_OK) || 90 (sk->sk_state == TCP_LISTEN || 91 sk->sk_state == TCP_CLOSE)) { 92 struct dctcp *ca = inet_csk_ca(sk); 93 94 ca->prior_snd_una = tp->snd_una; 95 ca->prior_rcv_nxt = tp->rcv_nxt; 96 97 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA); 98 99 ca->delayed_ack_reserved = 0; 100 ca->loss_cwnd = 0; 101 ca->ce_state = 0; 102 103 dctcp_reset(tp, ca); 104 return; 105 } 106 107 /* No ECN support? Fall back to Reno. Also need to clear 108 * ECT from sk since it is set during 3WHS for DCTCP. 109 */ 110 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 111 INET_ECN_dontxmit(sk); 112 } 113 114 static u32 dctcp_ssthresh(struct sock *sk) 115 { 116 struct dctcp *ca = inet_csk_ca(sk); 117 struct tcp_sock *tp = tcp_sk(sk); 118 119 ca->loss_cwnd = tp->snd_cwnd; 120 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 121 } 122 123 /* Minimal DCTP CE state machine: 124 * 125 * S: 0 <- last pkt was non-CE 126 * 1 <- last pkt was CE 127 */ 128 129 static void dctcp_ce_state_0_to_1(struct sock *sk) 130 { 131 struct dctcp *ca = inet_csk_ca(sk); 132 struct tcp_sock *tp = tcp_sk(sk); 133 134 /* State has changed from CE=0 to CE=1 and delayed 135 * ACK has not sent yet. 136 */ 137 if (!ca->ce_state && ca->delayed_ack_reserved) { 138 u32 tmp_rcv_nxt; 139 140 /* Save current rcv_nxt. */ 141 tmp_rcv_nxt = tp->rcv_nxt; 142 143 /* Generate previous ack with CE=0. */ 144 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; 145 tp->rcv_nxt = ca->prior_rcv_nxt; 146 147 tcp_send_ack(sk); 148 149 /* Recover current rcv_nxt. */ 150 tp->rcv_nxt = tmp_rcv_nxt; 151 } 152 153 ca->prior_rcv_nxt = tp->rcv_nxt; 154 ca->ce_state = 1; 155 156 tp->ecn_flags |= TCP_ECN_DEMAND_CWR; 157 } 158 159 static void dctcp_ce_state_1_to_0(struct sock *sk) 160 { 161 struct dctcp *ca = inet_csk_ca(sk); 162 struct tcp_sock *tp = tcp_sk(sk); 163 164 /* State has changed from CE=1 to CE=0 and delayed 165 * ACK has not sent yet. 166 */ 167 if (ca->ce_state && ca->delayed_ack_reserved) { 168 u32 tmp_rcv_nxt; 169 170 /* Save current rcv_nxt. */ 171 tmp_rcv_nxt = tp->rcv_nxt; 172 173 /* Generate previous ack with CE=1. */ 174 tp->ecn_flags |= TCP_ECN_DEMAND_CWR; 175 tp->rcv_nxt = ca->prior_rcv_nxt; 176 177 tcp_send_ack(sk); 178 179 /* Recover current rcv_nxt. */ 180 tp->rcv_nxt = tmp_rcv_nxt; 181 } 182 183 ca->prior_rcv_nxt = tp->rcv_nxt; 184 ca->ce_state = 0; 185 186 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; 187 } 188 189 static void dctcp_update_alpha(struct sock *sk, u32 flags) 190 { 191 const struct tcp_sock *tp = tcp_sk(sk); 192 struct dctcp *ca = inet_csk_ca(sk); 193 u32 acked_bytes = tp->snd_una - ca->prior_snd_una; 194 195 /* If ack did not advance snd_una, count dupack as MSS size. 196 * If ack did update window, do not count it at all. 197 */ 198 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE)) 199 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss; 200 if (acked_bytes) { 201 ca->acked_bytes_total += acked_bytes; 202 ca->prior_snd_una = tp->snd_una; 203 204 if (flags & CA_ACK_ECE) 205 ca->acked_bytes_ecn += acked_bytes; 206 } 207 208 /* Expired RTT */ 209 if (!before(tp->snd_una, ca->next_seq)) { 210 u64 bytes_ecn = ca->acked_bytes_ecn; 211 u32 alpha = ca->dctcp_alpha; 212 213 /* alpha = (1 - g) * alpha + g * F */ 214 215 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 216 if (bytes_ecn) { 217 /* If dctcp_shift_g == 1, a 32bit value would overflow 218 * after 8 Mbytes. 219 */ 220 bytes_ecn <<= (10 - dctcp_shift_g); 221 do_div(bytes_ecn, max(1U, ca->acked_bytes_total)); 222 223 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA); 224 } 225 /* dctcp_alpha can be read from dctcp_get_info() without 226 * synchro, so we ask compiler to not use dctcp_alpha 227 * as a temporary variable in prior operations. 228 */ 229 WRITE_ONCE(ca->dctcp_alpha, alpha); 230 dctcp_reset(tp, ca); 231 } 232 } 233 234 static void dctcp_state(struct sock *sk, u8 new_state) 235 { 236 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) { 237 struct dctcp *ca = inet_csk_ca(sk); 238 239 /* If this extension is enabled, we clamp dctcp_alpha to 240 * max on packet loss; the motivation is that dctcp_alpha 241 * is an indicator to the extend of congestion and packet 242 * loss is an indicator of extreme congestion; setting 243 * this in practice turned out to be beneficial, and 244 * effectively assumes total congestion which reduces the 245 * window by half. 246 */ 247 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 248 } 249 } 250 251 static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev) 252 { 253 struct dctcp *ca = inet_csk_ca(sk); 254 255 switch (ev) { 256 case CA_EVENT_DELAYED_ACK: 257 if (!ca->delayed_ack_reserved) 258 ca->delayed_ack_reserved = 1; 259 break; 260 case CA_EVENT_NON_DELAYED_ACK: 261 if (ca->delayed_ack_reserved) 262 ca->delayed_ack_reserved = 0; 263 break; 264 default: 265 /* Don't care for the rest. */ 266 break; 267 } 268 } 269 270 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 271 { 272 switch (ev) { 273 case CA_EVENT_ECN_IS_CE: 274 dctcp_ce_state_0_to_1(sk); 275 break; 276 case CA_EVENT_ECN_NO_CE: 277 dctcp_ce_state_1_to_0(sk); 278 break; 279 case CA_EVENT_DELAYED_ACK: 280 case CA_EVENT_NON_DELAYED_ACK: 281 dctcp_update_ack_reserved(sk, ev); 282 break; 283 default: 284 /* Don't care for the rest. */ 285 break; 286 } 287 } 288 289 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 290 union tcp_cc_info *info) 291 { 292 const struct dctcp *ca = inet_csk_ca(sk); 293 294 /* Fill it also in case of VEGASINFO due to req struct limits. 295 * We can still correctly retrieve it later. 296 */ 297 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 298 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 299 memset(&info->dctcp, 0, sizeof(info->dctcp)); 300 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 301 info->dctcp.dctcp_enabled = 1; 302 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 303 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 304 info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn; 305 info->dctcp.dctcp_ab_tot = ca->acked_bytes_total; 306 } 307 308 *attr = INET_DIAG_DCTCPINFO; 309 return sizeof(info->dctcp); 310 } 311 return 0; 312 } 313 314 static u32 dctcp_cwnd_undo(struct sock *sk) 315 { 316 const struct dctcp *ca = inet_csk_ca(sk); 317 318 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd); 319 } 320 321 static struct tcp_congestion_ops dctcp __read_mostly = { 322 .init = dctcp_init, 323 .in_ack_event = dctcp_update_alpha, 324 .cwnd_event = dctcp_cwnd_event, 325 .ssthresh = dctcp_ssthresh, 326 .cong_avoid = tcp_reno_cong_avoid, 327 .undo_cwnd = dctcp_cwnd_undo, 328 .set_state = dctcp_state, 329 .get_info = dctcp_get_info, 330 .flags = TCP_CONG_NEEDS_ECN, 331 .owner = THIS_MODULE, 332 .name = "dctcp", 333 }; 334 335 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 336 .ssthresh = tcp_reno_ssthresh, 337 .cong_avoid = tcp_reno_cong_avoid, 338 .undo_cwnd = tcp_reno_undo_cwnd, 339 .get_info = dctcp_get_info, 340 .owner = THIS_MODULE, 341 .name = "dctcp-reno", 342 }; 343 344 static int __init dctcp_register(void) 345 { 346 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 347 return tcp_register_congestion_control(&dctcp); 348 } 349 350 static void __exit dctcp_unregister(void) 351 { 352 tcp_unregister_congestion_control(&dctcp); 353 } 354 355 module_init(dctcp_register); 356 module_exit(dctcp_unregister); 357 358 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 359 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 360 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 361 362 MODULE_LICENSE("GPL v2"); 363 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 364