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 /* For avoiding denominator == 1. */ 208 if (ca->acked_bytes_total == 0) 209 ca->acked_bytes_total = 1; 210 211 /* alpha = (1 - g) * alpha + g * F */ 212 ca->dctcp_alpha = ca->dctcp_alpha - 213 (ca->dctcp_alpha >> dctcp_shift_g) + 214 (ca->acked_bytes_ecn << (10U - dctcp_shift_g)) / 215 ca->acked_bytes_total; 216 217 if (ca->dctcp_alpha > DCTCP_MAX_ALPHA) 218 /* Clamp dctcp_alpha to max. */ 219 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 220 221 dctcp_reset(tp, ca); 222 } 223 } 224 225 static void dctcp_state(struct sock *sk, u8 new_state) 226 { 227 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) { 228 struct dctcp *ca = inet_csk_ca(sk); 229 230 /* If this extension is enabled, we clamp dctcp_alpha to 231 * max on packet loss; the motivation is that dctcp_alpha 232 * is an indicator to the extend of congestion and packet 233 * loss is an indicator of extreme congestion; setting 234 * this in practice turned out to be beneficial, and 235 * effectively assumes total congestion which reduces the 236 * window by half. 237 */ 238 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 239 } 240 } 241 242 static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev) 243 { 244 struct dctcp *ca = inet_csk_ca(sk); 245 246 switch (ev) { 247 case CA_EVENT_DELAYED_ACK: 248 if (!ca->delayed_ack_reserved) 249 ca->delayed_ack_reserved = 1; 250 break; 251 case CA_EVENT_NON_DELAYED_ACK: 252 if (ca->delayed_ack_reserved) 253 ca->delayed_ack_reserved = 0; 254 break; 255 default: 256 /* Don't care for the rest. */ 257 break; 258 } 259 } 260 261 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 262 { 263 switch (ev) { 264 case CA_EVENT_ECN_IS_CE: 265 dctcp_ce_state_0_to_1(sk); 266 break; 267 case CA_EVENT_ECN_NO_CE: 268 dctcp_ce_state_1_to_0(sk); 269 break; 270 case CA_EVENT_DELAYED_ACK: 271 case CA_EVENT_NON_DELAYED_ACK: 272 dctcp_update_ack_reserved(sk, ev); 273 break; 274 default: 275 /* Don't care for the rest. */ 276 break; 277 } 278 } 279 280 static void dctcp_get_info(struct sock *sk, u32 ext, struct sk_buff *skb) 281 { 282 const struct dctcp *ca = inet_csk_ca(sk); 283 284 /* Fill it also in case of VEGASINFO due to req struct limits. 285 * We can still correctly retrieve it later. 286 */ 287 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) || 288 ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 289 struct tcp_dctcp_info info; 290 291 memset(&info, 0, sizeof(info)); 292 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) { 293 info.dctcp_enabled = 1; 294 info.dctcp_ce_state = (u16) ca->ce_state; 295 info.dctcp_alpha = ca->dctcp_alpha; 296 info.dctcp_ab_ecn = ca->acked_bytes_ecn; 297 info.dctcp_ab_tot = ca->acked_bytes_total; 298 } 299 300 nla_put(skb, INET_DIAG_DCTCPINFO, sizeof(info), &info); 301 } 302 } 303 304 static struct tcp_congestion_ops dctcp __read_mostly = { 305 .init = dctcp_init, 306 .in_ack_event = dctcp_update_alpha, 307 .cwnd_event = dctcp_cwnd_event, 308 .ssthresh = dctcp_ssthresh, 309 .cong_avoid = tcp_reno_cong_avoid, 310 .set_state = dctcp_state, 311 .get_info = dctcp_get_info, 312 .flags = TCP_CONG_NEEDS_ECN, 313 .owner = THIS_MODULE, 314 .name = "dctcp", 315 }; 316 317 static struct tcp_congestion_ops dctcp_reno __read_mostly = { 318 .ssthresh = tcp_reno_ssthresh, 319 .cong_avoid = tcp_reno_cong_avoid, 320 .get_info = dctcp_get_info, 321 .owner = THIS_MODULE, 322 .name = "dctcp-reno", 323 }; 324 325 static int __init dctcp_register(void) 326 { 327 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE); 328 return tcp_register_congestion_control(&dctcp); 329 } 330 331 static void __exit dctcp_unregister(void) 332 { 333 tcp_unregister_congestion_control(&dctcp); 334 } 335 336 module_init(dctcp_register); 337 module_exit(dctcp_unregister); 338 339 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>"); 340 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>"); 341 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>"); 342 343 MODULE_LICENSE("GPL v2"); 344 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)"); 345