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 55 /* There are several situations when we must "re-start" Vegas: 56 * 57 * o when a connection is established 58 * o after an RTO 59 * o after fast recovery 60 * o when we send a packet and there is no outstanding 61 * unacknowledged data (restarting an idle connection) 62 * 63 * In these circumstances we cannot do a Vegas calculation at the 64 * end of the first RTT, because any calculation we do is using 65 * stale info -- both the saved cwnd and congestion feedback are 66 * stale. 67 * 68 * Instead we must wait until the completion of an RTT during 69 * which we actually receive ACKs. 70 */ 71 static void vegas_enable(struct sock *sk) 72 { 73 const struct tcp_sock *tp = tcp_sk(sk); 74 struct vegas *vegas = inet_csk_ca(sk); 75 76 /* Begin taking Vegas samples next time we send something. */ 77 vegas->doing_vegas_now = 1; 78 79 /* Set the beginning of the next send window. */ 80 vegas->beg_snd_nxt = tp->snd_nxt; 81 82 vegas->cntRTT = 0; 83 vegas->minRTT = 0x7fffffff; 84 } 85 86 /* Stop taking Vegas samples for now. */ 87 static inline void vegas_disable(struct sock *sk) 88 { 89 struct vegas *vegas = inet_csk_ca(sk); 90 91 vegas->doing_vegas_now = 0; 92 } 93 94 void tcp_vegas_init(struct sock *sk) 95 { 96 struct vegas *vegas = inet_csk_ca(sk); 97 98 vegas->baseRTT = 0x7fffffff; 99 vegas_enable(sk); 100 } 101 EXPORT_SYMBOL_GPL(tcp_vegas_init); 102 103 /* Do RTT sampling needed for Vegas. 104 * Basically we: 105 * o min-filter RTT samples from within an RTT to get the current 106 * propagation delay + queuing delay (we are min-filtering to try to 107 * avoid the effects of delayed ACKs) 108 * o min-filter RTT samples from a much longer window (forever for now) 109 * to find the propagation delay (baseRTT) 110 */ 111 void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us) 112 { 113 struct vegas *vegas = inet_csk_ca(sk); 114 u32 vrtt; 115 116 if (rtt_us < 0) 117 return; 118 119 /* Never allow zero rtt or baseRTT */ 120 vrtt = rtt_us + 1; 121 122 /* Filter to find propagation delay: */ 123 if (vrtt < vegas->baseRTT) 124 vegas->baseRTT = vrtt; 125 126 /* Find the min RTT during the last RTT to find 127 * the current prop. delay + queuing delay: 128 */ 129 vegas->minRTT = min(vegas->minRTT, vrtt); 130 vegas->cntRTT++; 131 } 132 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked); 133 134 void tcp_vegas_state(struct sock *sk, u8 ca_state) 135 { 136 137 if (ca_state == TCP_CA_Open) 138 vegas_enable(sk); 139 else 140 vegas_disable(sk); 141 } 142 EXPORT_SYMBOL_GPL(tcp_vegas_state); 143 144 /* 145 * If the connection is idle and we are restarting, 146 * then we don't want to do any Vegas calculations 147 * until we get fresh RTT samples. So when we 148 * restart, we reset our Vegas state to a clean 149 * slate. After we get acks for this flight of 150 * packets, _then_ we can make Vegas calculations 151 * again. 152 */ 153 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) 154 { 155 if (event == CA_EVENT_CWND_RESTART || 156 event == CA_EVENT_TX_START) 157 tcp_vegas_init(sk); 158 } 159 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event); 160 161 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp) 162 { 163 return min(tp->snd_ssthresh, tp->snd_cwnd-1); 164 } 165 166 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight) 167 { 168 struct tcp_sock *tp = tcp_sk(sk); 169 struct vegas *vegas = inet_csk_ca(sk); 170 171 if (!vegas->doing_vegas_now) { 172 tcp_reno_cong_avoid(sk, ack, in_flight); 173 return; 174 } 175 176 if (after(ack, vegas->beg_snd_nxt)) { 177 /* Do the Vegas once-per-RTT cwnd adjustment. */ 178 179 /* Save the extent of the current window so we can use this 180 * at the end of the next RTT. 181 */ 182 vegas->beg_snd_nxt = tp->snd_nxt; 183 184 /* We do the Vegas calculations only if we got enough RTT 185 * samples that we can be reasonably sure that we got 186 * at least one RTT sample that wasn't from a delayed ACK. 187 * If we only had 2 samples total, 188 * then that means we're getting only 1 ACK per RTT, which 189 * means they're almost certainly delayed ACKs. 190 * If we have 3 samples, we should be OK. 191 */ 192 193 if (vegas->cntRTT <= 2) { 194 /* We don't have enough RTT samples to do the Vegas 195 * calculation, so we'll behave like Reno. 196 */ 197 tcp_reno_cong_avoid(sk, ack, in_flight); 198 } else { 199 u32 rtt, diff; 200 u64 target_cwnd; 201 202 /* We have enough RTT samples, so, using the Vegas 203 * algorithm, we determine if we should increase or 204 * decrease cwnd, and by how much. 205 */ 206 207 /* Pluck out the RTT we are using for the Vegas 208 * calculations. This is the min RTT seen during the 209 * last RTT. Taking the min filters out the effects 210 * of delayed ACKs, at the cost of noticing congestion 211 * a bit later. 212 */ 213 rtt = vegas->minRTT; 214 215 /* Calculate the cwnd we should have, if we weren't 216 * going too fast. 217 * 218 * This is: 219 * (actual rate in segments) * baseRTT 220 */ 221 target_cwnd = tp->snd_cwnd * vegas->baseRTT / rtt; 222 223 /* Calculate the difference between the window we had, 224 * and the window we would like to have. This quantity 225 * is the "Diff" from the Arizona Vegas papers. 226 */ 227 diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT; 228 229 if (diff > gamma && tp->snd_cwnd <= tp->snd_ssthresh) { 230 /* Going too fast. Time to slow down 231 * and switch to congestion avoidance. 232 */ 233 234 /* Set cwnd to match the actual rate 235 * exactly: 236 * cwnd = (actual rate) * baseRTT 237 * Then we add 1 because the integer 238 * truncation robs us of full link 239 * utilization. 240 */ 241 tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1); 242 tp->snd_ssthresh = tcp_vegas_ssthresh(tp); 243 244 } else if (tp->snd_cwnd <= tp->snd_ssthresh) { 245 /* Slow start. */ 246 tcp_slow_start(tp); 247 } else { 248 /* Congestion avoidance. */ 249 250 /* Figure out where we would like cwnd 251 * to be. 252 */ 253 if (diff > beta) { 254 /* The old window was too fast, so 255 * we slow down. 256 */ 257 tp->snd_cwnd--; 258 tp->snd_ssthresh 259 = tcp_vegas_ssthresh(tp); 260 } else if (diff < alpha) { 261 /* We don't have enough extra packets 262 * in the network, so speed up. 263 */ 264 tp->snd_cwnd++; 265 } else { 266 /* Sending just as fast as we 267 * should be. 268 */ 269 } 270 } 271 272 if (tp->snd_cwnd < 2) 273 tp->snd_cwnd = 2; 274 else if (tp->snd_cwnd > tp->snd_cwnd_clamp) 275 tp->snd_cwnd = tp->snd_cwnd_clamp; 276 277 tp->snd_ssthresh = tcp_current_ssthresh(sk); 278 } 279 280 /* Wipe the slate clean for the next RTT. */ 281 vegas->cntRTT = 0; 282 vegas->minRTT = 0x7fffffff; 283 } 284 /* Use normal slow start */ 285 else if (tp->snd_cwnd <= tp->snd_ssthresh) 286 tcp_slow_start(tp); 287 288 } 289 290 /* Extract info for Tcp socket info provided via netlink. */ 291 void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb) 292 { 293 const struct vegas *ca = inet_csk_ca(sk); 294 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 295 struct tcpvegas_info info = { 296 .tcpv_enabled = ca->doing_vegas_now, 297 .tcpv_rttcnt = ca->cntRTT, 298 .tcpv_rtt = ca->baseRTT, 299 .tcpv_minrtt = ca->minRTT, 300 }; 301 302 nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info); 303 } 304 } 305 EXPORT_SYMBOL_GPL(tcp_vegas_get_info); 306 307 static struct tcp_congestion_ops tcp_vegas __read_mostly = { 308 .flags = TCP_CONG_RTT_STAMP, 309 .init = tcp_vegas_init, 310 .ssthresh = tcp_reno_ssthresh, 311 .cong_avoid = tcp_vegas_cong_avoid, 312 .min_cwnd = tcp_reno_min_cwnd, 313 .pkts_acked = tcp_vegas_pkts_acked, 314 .set_state = tcp_vegas_state, 315 .cwnd_event = tcp_vegas_cwnd_event, 316 .get_info = tcp_vegas_get_info, 317 318 .owner = THIS_MODULE, 319 .name = "vegas", 320 }; 321 322 static int __init tcp_vegas_register(void) 323 { 324 BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); 325 tcp_register_congestion_control(&tcp_vegas); 326 return 0; 327 } 328 329 static void __exit tcp_vegas_unregister(void) 330 { 331 tcp_unregister_congestion_control(&tcp_vegas); 332 } 333 334 module_init(tcp_vegas_register); 335 module_exit(tcp_vegas_unregister); 336 337 MODULE_AUTHOR("Stephen Hemminger"); 338 MODULE_LICENSE("GPL"); 339 MODULE_DESCRIPTION("TCP Vegas"); 340