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 }; 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->delayed_ack_reserved = 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 const struct dctcp *ca = inet_csk_ca(sk); 115 struct tcp_sock *tp = tcp_sk(sk); 116 117 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 118 } 119 120 /* Minimal DCTP CE state machine: 121 * 122 * S: 0 <- last pkt was non-CE 123 * 1 <- last pkt was CE 124 */ 125 126 static void dctcp_ce_state_0_to_1(struct sock *sk) 127 { 128 struct dctcp *ca = inet_csk_ca(sk); 129 struct tcp_sock *tp = tcp_sk(sk); 130 131 /* State has changed from CE=0 to CE=1 and delayed 132 * ACK has not sent yet. 133 */ 134 if (!ca->ce_state && ca->delayed_ack_reserved) { 135 u32 tmp_rcv_nxt; 136 137 /* Save current rcv_nxt. */ 138 tmp_rcv_nxt = tp->rcv_nxt; 139 140 /* Generate previous ack with CE=0. */ 141 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; 142 tp->rcv_nxt = ca->prior_rcv_nxt; 143 144 tcp_send_ack(sk); 145 146 /* Recover current rcv_nxt. */ 147 tp->rcv_nxt = tmp_rcv_nxt; 148 } 149 150 ca->prior_rcv_nxt = tp->rcv_nxt; 151 ca->ce_state = 1; 152 153 tp->ecn_flags |= TCP_ECN_DEMAND_CWR; 154 } 155 156 static void dctcp_ce_state_1_to_0(struct sock *sk) 157 { 158 struct dctcp *ca = inet_csk_ca(sk); 159 struct tcp_sock *tp = tcp_sk(sk); 160 161 /* State has changed from CE=1 to CE=0 and delayed 162 * ACK has not sent yet. 163 */ 164 if (ca->ce_state && ca->delayed_ack_reserved) { 165 u32 tmp_rcv_nxt; 166 167 /* Save current rcv_nxt. */ 168 tmp_rcv_nxt = tp->rcv_nxt; 169 170 /* Generate previous ack with CE=1. */ 171 tp->ecn_flags |= TCP_ECN_DEMAND_CWR; 172 tp->rcv_nxt = ca->prior_rcv_nxt; 173 174 tcp_send_ack(sk); 175 176 /* Recover current rcv_nxt. */ 177 tp->rcv_nxt = tmp_rcv_nxt; 178 } 179 180 ca->prior_rcv_nxt = tp->rcv_nxt; 181 ca->ce_state = 0; 182 183 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR; 184 } 185 186 static void dctcp_update_alpha(struct sock *sk, u32 flags) 187 { 188 const struct tcp_sock *tp = tcp_sk(sk); 189 struct dctcp *ca = inet_csk_ca(sk); 190 u32 acked_bytes = tp->snd_una - ca->prior_snd_una; 191 192 /* If ack did not advance snd_una, count dupack as MSS size. 193 * If ack did update window, do not count it at all. 194 */ 195 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE)) 196 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss; 197 if (acked_bytes) { 198 ca->acked_bytes_total += acked_bytes; 199 ca->prior_snd_una = tp->snd_una; 200 201 if (flags & CA_ACK_ECE) 202 ca->acked_bytes_ecn += acked_bytes; 203 } 204 205 /* Expired RTT */ 206 if (!before(tp->snd_una, ca->next_seq)) { 207 u64 bytes_ecn = ca->acked_bytes_ecn; 208 u32 alpha = ca->dctcp_alpha; 209 210 /* alpha = (1 - g) * alpha + g * F */ 211 212 alpha -= alpha >> dctcp_shift_g; 213 if (bytes_ecn) { 214 /* If dctcp_shift_g == 1, a 32bit value would overflow 215 * after 8 Mbytes. 216 */ 217 bytes_ecn <<= (10 - dctcp_shift_g); 218 do_div(bytes_ecn, max(1U, ca->acked_bytes_total)); 219 220 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA); 221 } 222 /* dctcp_alpha can be read from dctcp_get_info() without 223 * synchro, so we ask compiler to not use dctcp_alpha 224 * as a temporary variable in prior operations. 225 */ 226 WRITE_ONCE(ca->dctcp_alpha, alpha); 227 dctcp_reset(tp, ca); 228 } 229 } 230 231 static void dctcp_state(struct sock *sk, u8 new_state) 232 { 233 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) { 234 struct dctcp *ca = inet_csk_ca(sk); 235 236 /* If this extension is enabled, we clamp dctcp_alpha to 237 * max on packet loss; the motivation is that dctcp_alpha 238 * is an indicator to the extend of congestion and packet 239 * loss is an indicator of extreme congestion; setting 240 * this in practice turned out to be beneficial, and 241 * effectively assumes total congestion which reduces the 242 * window by half. 243 */ 244 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 245 } 246 } 247 248 static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev) 249 { 250 struct dctcp *ca = inet_csk_ca(sk); 251 252 switch (ev) { 253 case CA_EVENT_DELAYED_ACK: 254 if (!ca->delayed_ack_reserved) 255 ca->delayed_ack_reserved = 1; 256 break; 257 case CA_EVENT_NON_DELAYED_ACK: 258 if (ca->delayed_ack_reserved) 259 ca->delayed_ack_reserved = 0; 260 break; 261 default: 262 /* Don't care for the rest. */ 263 break; 264 } 265 } 266 267 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 268 { 269 switch (ev) { 270 case CA_EVENT_ECN_IS_CE: 271 dctcp_ce_state_0_to_1(sk); 272 break; 273 case CA_EVENT_ECN_NO_CE: 274 dctcp_ce_state_1_to_0(sk); 275 break; 276 case CA_EVENT_DELAYED_ACK: 277 case CA_EVENT_NON_DELAYED_ACK: 278 dctcp_update_ack_reserved(sk, ev); 279 break; 280 default: 281 /* Don't care for the rest. */ 282 break; 283 } 284 } 285 286 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr, 287 union tcp_cc_info *info) 288 { 289 const struct dctcp *ca = inet_csk_ca(sk); 290 291 /* Fill it also in case of VEGASINFO due to req struct limits. 292 * We can still correctly retrieve it later. 293 */ 294 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 295 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 296 memset(info, 0, sizeof(struct tcp_dctcp_info)); 297 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 298 info->dctcp.dctcp_enabled = 1; 299 info->dctcp.dctcp_ce_state = (u16) ca->ce_state; 300 info->dctcp.dctcp_alpha = ca->dctcp_alpha; 301 info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn; 302 info->dctcp.dctcp_ab_tot = ca->acked_bytes_total; 303 } 304 305 *attr = INET_DIAG_DCTCPINFO; 306 return sizeof(*info); 307 } 308 return 0; 309 } 310 311 static struct tcp_congestion_ops dctcp __read_mostly = { 312 .init = dctcp_init, 313 .in_ack_event = dctcp_update_alpha, 314 .cwnd_event = dctcp_cwnd_event, 315 .ssthresh = dctcp_ssthresh, 316 .cong_avoid = tcp_reno_cong_avoid, 317 .set_state = dctcp_state, 318 .get_info = dctcp_get_info, 319 .flags = TCP_CONG_NEEDS_ECN, 320 .owner = THIS_MODULE, 321 .name = "dctcp", 322 }; 323 324 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 325 .ssthresh = tcp_reno_ssthresh, 326 .cong_avoid = tcp_reno_cong_avoid, 327 .get_info = dctcp_get_info, 328 .owner = THIS_MODULE, 329 .name = "dctcp-reno", 330 }; 331 332 static int __init dctcp_register(void) 333 { 334 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 335 return tcp_register_congestion_control(&dctcp); 336 } 337 338 static void __exit dctcp_unregister(void) 339 { 340 tcp_unregister_congestion_control(&dctcp); 341 } 342 343 module_init(dctcp_register); 344 module_exit(dctcp_unregister); 345 346 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 347 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 348 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 349 350 MODULE_LICENSE("GPL v2"); 351 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 352