1 /* 2 * TCP Vegas congestion control 3 * 4 * This is based on the congestion detection/avoidance scheme described in 5 * Lawrence S. Brakmo and Larry L. Peterson. 6 * "TCP Vegas: End to end congestion avoidance on a global internet." 7 * IEEE Journal on Selected Areas in Communication, 13(8):1465--1480, 8 * October 1995. Available from: 9 * ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps 10 * 11 * See http://www.cs.arizona.edu/xkernel/ for their implementation. 12 * The main aspects that distinguish this implementation from the 13 * Arizona Vegas implementation are: 14 * o We do not change the loss detection or recovery mechanisms of 15 * Linux in any way. Linux already recovers from losses quite well, 16 * using fine-grained timers, NewReno, and FACK. 17 * o To avoid the performance penalty imposed by increasing cwnd 18 * only every-other RTT during slow start, we increase during 19 * every RTT during slow start, just like Reno. 20 * o Largely to allow continuous cwnd growth during slow start, 21 * we use the rate at which ACKs come back as the "actual" 22 * rate, rather than the rate at which data is sent. 23 * o To speed convergence to the right rate, we set the cwnd 24 * to achieve the right ("actual") rate when we exit slow start. 25 * o To filter out the noise caused by delayed ACKs, we use the 26 * minimum RTT sample observed during the last RTT to calculate 27 * the actual rate. 28 * o When the sender re-starts from idle, it waits until it has 29 * received ACKs for an entire flight of new data before making 30 * a cwnd adjustment decision. The original Vegas implementation 31 * assumed senders never went idle. 32 */ 33 34 #include <linux/mm.h> 35 #include <linux/module.h> 36 #include <linux/skbuff.h> 37 #include <linux/inet_diag.h> 38 39 #include <net/tcp.h> 40 41 #include "tcp_vegas.h" 42 43 static int alpha = 2; 44 static int beta = 4; 45 static int gamma = 1; 46 47 module_param(alpha, int, 0644); 48 MODULE_PARM_DESC(alpha, "lower bound of packets in network"); 49 module_param(beta, int, 0644); 50 MODULE_PARM_DESC(beta, "upper bound of packets in network"); 51 module_param(gamma, int, 0644); 52 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); 53 54 /* There are several situations when we must "re-start" Vegas: 55 * 56 * o when a connection is established 57 * o after an RTO 58 * o after fast recovery 59 * o when we send a packet and there is no outstanding 60 * unacknowledged data (restarting an idle connection) 61 * 62 * In these circumstances we cannot do a Vegas calculation at the 63 * end of the first RTT, because any calculation we do is using 64 * stale info -- both the saved cwnd and congestion feedback are 65 * stale. 66 * 67 * Instead we must wait until the completion of an RTT during 68 * which we actually receive ACKs. 69 */ 70 static void vegas_enable(struct sock *sk) 71 { 72 const struct tcp_sock *tp = tcp_sk(sk); 73 struct vegas *vegas = inet_csk_ca(sk); 74 75 /* Begin taking Vegas samples next time we send something. */ 76 vegas->doing_vegas_now = 1; 77 78 /* Set the beginning of the next send window. */ 79 vegas->beg_snd_nxt = tp->snd_nxt; 80 81 vegas->cntRTT = 0; 82 vegas->minRTT = 0x7fffffff; 83 } 84 85 /* Stop taking Vegas samples for now. */ 86 static inline void vegas_disable(struct sock *sk) 87 { 88 struct vegas *vegas = inet_csk_ca(sk); 89 90 vegas->doing_vegas_now = 0; 91 } 92 93 void tcp_vegas_init(struct sock *sk) 94 { 95 struct vegas *vegas = inet_csk_ca(sk); 96 97 vegas->baseRTT = 0x7fffffff; 98 vegas_enable(sk); 99 } 100 EXPORT_SYMBOL_GPL(tcp_vegas_init); 101 102 /* Do RTT sampling needed for Vegas. 103 * Basically we: 104 * o min-filter RTT samples from within an RTT to get the current 105 * propagation delay + queuing delay (we are min-filtering to try to 106 * avoid the effects of delayed ACKs) 107 * o min-filter RTT samples from a much longer window (forever for now) 108 * to find the propagation delay (baseRTT) 109 */ 110 void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample) 111 { 112 struct vegas *vegas = inet_csk_ca(sk); 113 u32 vrtt; 114 115 if (sample->rtt_us < 0) 116 return; 117 118 /* Never allow zero rtt or baseRTT */ 119 vrtt = sample->rtt_us + 1; 120 121 /* Filter to find propagation delay: */ 122 if (vrtt < vegas->baseRTT) 123 vegas->baseRTT = vrtt; 124 125 /* Find the min RTT during the last RTT to find 126 * the current prop. delay + queuing delay: 127 */ 128 vegas->minRTT = min(vegas->minRTT, vrtt); 129 vegas->cntRTT++; 130 } 131 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked); 132 133 void tcp_vegas_state(struct sock *sk, u8 ca_state) 134 { 135 if (ca_state == TCP_CA_Open) 136 vegas_enable(sk); 137 else 138 vegas_disable(sk); 139 } 140 EXPORT_SYMBOL_GPL(tcp_vegas_state); 141 142 /* 143 * If the connection is idle and we are restarting, 144 * then we don't want to do any Vegas calculations 145 * until we get fresh RTT samples. So when we 146 * restart, we reset our Vegas state to a clean 147 * slate. After we get acks for this flight of 148 * packets, _then_ we can make Vegas calculations 149 * again. 150 */ 151 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) 152 { 153 if (event == CA_EVENT_CWND_RESTART || 154 event == CA_EVENT_TX_START) 155 tcp_vegas_init(sk); 156 } 157 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event); 158 159 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp) 160 { 161 return min(tp->snd_ssthresh, tp->snd_cwnd-1); 162 } 163 164 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked) 165 { 166 struct tcp_sock *tp = tcp_sk(sk); 167 struct vegas *vegas = inet_csk_ca(sk); 168 169 if (!vegas->doing_vegas_now) { 170 tcp_reno_cong_avoid(sk, ack, acked); 171 return; 172 } 173 174 if (after(ack, vegas->beg_snd_nxt)) { 175 /* Do the Vegas once-per-RTT cwnd adjustment. */ 176 177 /* Save the extent of the current window so we can use this 178 * at the end of the next RTT. 179 */ 180 vegas->beg_snd_nxt = tp->snd_nxt; 181 182 /* We do the Vegas calculations only if we got enough RTT 183 * samples that we can be reasonably sure that we got 184 * at least one RTT sample that wasn't from a delayed ACK. 185 * If we only had 2 samples total, 186 * then that means we're getting only 1 ACK per RTT, which 187 * means they're almost certainly delayed ACKs. 188 * If we have 3 samples, we should be OK. 189 */ 190 191 if (vegas->cntRTT <= 2) { 192 /* We don't have enough RTT samples to do the Vegas 193 * calculation, so we'll behave like Reno. 194 */ 195 tcp_reno_cong_avoid(sk, ack, acked); 196 } else { 197 u32 rtt, diff; 198 u64 target_cwnd; 199 200 /* We have enough RTT samples, so, using the Vegas 201 * algorithm, we determine if we should increase or 202 * decrease cwnd, and by how much. 203 */ 204 205 /* Pluck out the RTT we are using for the Vegas 206 * calculations. This is the min RTT seen during the 207 * last RTT. Taking the min filters out the effects 208 * of delayed ACKs, at the cost of noticing congestion 209 * a bit later. 210 */ 211 rtt = vegas->minRTT; 212 213 /* Calculate the cwnd we should have, if we weren't 214 * going too fast. 215 * 216 * This is: 217 * (actual rate in segments) * baseRTT 218 */ 219 target_cwnd = (u64)tp->snd_cwnd * vegas->baseRTT; 220 do_div(target_cwnd, rtt); 221 222 /* Calculate the difference between the window we had, 223 * and the window we would like to have. This quantity 224 * is the "Diff" from the Arizona Vegas papers. 225 */ 226 diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT; 227 228 if (diff > gamma && tcp_in_slow_start(tp)) { 229 /* Going too fast. Time to slow down 230 * and switch to congestion avoidance. 231 */ 232 233 /* Set cwnd to match the actual rate 234 * exactly: 235 * cwnd = (actual rate) * baseRTT 236 * Then we add 1 because the integer 237 * truncation robs us of full link 238 * utilization. 239 */ 240 tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1); 241 tp->snd_ssthresh = tcp_vegas_ssthresh(tp); 242 243 } else if (tcp_in_slow_start(tp)) { 244 /* Slow start. */ 245 tcp_slow_start(tp, acked); 246 } else { 247 /* Congestion avoidance. */ 248 249 /* Figure out where we would like cwnd 250 * to be. 251 */ 252 if (diff > beta) { 253 /* The old window was too fast, so 254 * we slow down. 255 */ 256 tp->snd_cwnd--; 257 tp->snd_ssthresh 258 = tcp_vegas_ssthresh(tp); 259 } else if (diff < alpha) { 260 /* We don't have enough extra packets 261 * in the network, so speed up. 262 */ 263 tp->snd_cwnd++; 264 } else { 265 /* Sending just as fast as we 266 * should be. 267 */ 268 } 269 } 270 271 if (tp->snd_cwnd < 2) 272 tp->snd_cwnd = 2; 273 else if (tp->snd_cwnd > tp->snd_cwnd_clamp) 274 tp->snd_cwnd = tp->snd_cwnd_clamp; 275 276 tp->snd_ssthresh = tcp_current_ssthresh(sk); 277 } 278 279 /* Wipe the slate clean for the next RTT. */ 280 vegas->cntRTT = 0; 281 vegas->minRTT = 0x7fffffff; 282 } 283 /* Use normal slow start */ 284 else if (tcp_in_slow_start(tp)) 285 tcp_slow_start(tp, acked); 286 } 287 288 /* Extract info for Tcp socket info provided via netlink. */ 289 size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr, 290 union tcp_cc_info *info) 291 { 292 const struct vegas *ca = inet_csk_ca(sk); 293 294 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 295 info->vegas.tcpv_enabled = ca->doing_vegas_now, 296 info->vegas.tcpv_rttcnt = ca->cntRTT, 297 info->vegas.tcpv_rtt = ca->baseRTT, 298 info->vegas.tcpv_minrtt = ca->minRTT, 299 300 *attr = INET_DIAG_VEGASINFO; 301 return sizeof(struct tcpvegas_info); 302 } 303 return 0; 304 } 305 EXPORT_SYMBOL_GPL(tcp_vegas_get_info); 306 307 static struct tcp_congestion_ops tcp_vegas __read_mostly = { 308 .init = tcp_vegas_init, 309 .ssthresh = tcp_reno_ssthresh, 310 .undo_cwnd = tcp_reno_undo_cwnd, 311 .cong_avoid = tcp_vegas_cong_avoid, 312 .pkts_acked = tcp_vegas_pkts_acked, 313 .set_state = tcp_vegas_state, 314 .cwnd_event = tcp_vegas_cwnd_event, 315 .get_info = tcp_vegas_get_info, 316 317 .owner = THIS_MODULE, 318 .name = "vegas", 319 }; 320 321 static int __init tcp_vegas_register(void) 322 { 323 BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); 324 tcp_register_congestion_control(&tcp_vegas); 325 return 0; 326 } 327 328 static void __exit tcp_vegas_unregister(void) 329 { 330 tcp_unregister_congestion_control(&tcp_vegas); 331 } 332 333 module_init(tcp_vegas_register); 334 module_exit(tcp_vegas_unregister); 335 336 MODULE_AUTHOR("Stephen Hemminger"); 337 MODULE_LICENSE("GPL"); 338 MODULE_DESCRIPTION("TCP Vegas"); 339