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 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->loss_cwnd = 0; 100 ca->ce_state = 0; 101 102 dctcp_reset(tp, ca); 103 return; 104 } 105 106 /* No ECN support? Fall back to Reno. Also need to clear 107 * ECT from sk since it is set during 3WHS for DCTCP. 108 */ 109 inet_csk(sk)->icsk_ca_ops = &dctcp_reno; 110 INET_ECN_dontxmit(sk); 111 } 112 113 static u32 dctcp_ssthresh(struct sock *sk) 114 { 115 struct dctcp *ca = inet_csk_ca(sk); 116 struct tcp_sock *tp = tcp_sk(sk); 117 118 ca->loss_cwnd = tp->snd_cwnd; 119 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U); 120 } 121 122 static void dctcp_update_alpha(struct sock *sk, u32 flags) 123 { 124 const struct tcp_sock *tp = tcp_sk(sk); 125 struct dctcp *ca = inet_csk_ca(sk); 126 u32 acked_bytes = tp->snd_una - ca->prior_snd_una; 127 128 /* If ack did not advance snd_una, count dupack as MSS size. 129 * If ack did update window, do not count it at all. 130 */ 131 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE)) 132 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss; 133 if (acked_bytes) { 134 ca->acked_bytes_total += acked_bytes; 135 ca->prior_snd_una = tp->snd_una; 136 137 if (flags & CA_ACK_ECE) 138 ca->acked_bytes_ecn += acked_bytes; 139 } 140 141 /* Expired RTT */ 142 if (!before(tp->snd_una, ca->next_seq)) { 143 u64 bytes_ecn = ca->acked_bytes_ecn; 144 u32 alpha = ca->dctcp_alpha; 145 146 /* alpha = (1 - g) * alpha + g * F */ 147 148 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g); 149 if (bytes_ecn) { 150 /* If dctcp_shift_g == 1, a 32bit value would overflow 151 * after 8 Mbytes. 152 */ 153 bytes_ecn <<= (10 - dctcp_shift_g); 154 do_div(bytes_ecn, max(1U, ca->acked_bytes_total)); 155 156 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA); 157 } 158 /* dctcp_alpha can be read from dctcp_get_info() without 159 * synchro, so we ask compiler to not use dctcp_alpha 160 * as a temporary variable in prior operations. 161 */ 162 WRITE_ONCE(ca->dctcp_alpha, alpha); 163 dctcp_reset(tp, ca); 164 } 165 } 166 167 static void dctcp_state(struct sock *sk, u8 new_state) 168 { 169 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) { 170 struct dctcp *ca = inet_csk_ca(sk); 171 172 /* If this extension is enabled, we clamp dctcp_alpha to 173 * max on packet loss; the motivation is that dctcp_alpha 174 * is an indicator to the extend of congestion and packet 175 * loss is an indicator of extreme congestion; setting 176 * this in practice turned out to be beneficial, and 177 * effectively assumes total congestion which reduces the 178 * window by half. 179 */ 180 ca->dctcp_alpha = DCTCP_MAX_ALPHA; 181 } 182 } 183 184 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev) 185 { 186 struct dctcp *ca = inet_csk_ca(sk); 187 188 switch (ev) { 189 case CA_EVENT_ECN_IS_CE: 190 case CA_EVENT_ECN_NO_CE: 191 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state); 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