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/config.h> 35 #include <linux/mm.h> 36 #include <linux/module.h> 37 #include <linux/skbuff.h> 38 #include <linux/inet_diag.h> 39 40 #include <net/tcp.h> 41 42 /* Default values of the Vegas variables, in fixed-point representation 43 * with V_PARAM_SHIFT bits to the right of the binary point. 44 */ 45 #define V_PARAM_SHIFT 1 46 static int alpha = 1<<V_PARAM_SHIFT; 47 static int beta = 3<<V_PARAM_SHIFT; 48 static int gamma = 1<<V_PARAM_SHIFT; 49 50 module_param(alpha, int, 0644); 51 MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)"); 52 module_param(beta, int, 0644); 53 MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)"); 54 module_param(gamma, int, 0644); 55 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); 56 57 58 /* Vegas variables */ 59 struct vegas { 60 u32 beg_snd_nxt; /* right edge during last RTT */ 61 u32 beg_snd_una; /* left edge during last RTT */ 62 u32 beg_snd_cwnd; /* saves the size of the cwnd */ 63 u8 doing_vegas_now;/* if true, do vegas for this RTT */ 64 u16 cntRTT; /* # of RTTs measured within last RTT */ 65 u32 minRTT; /* min of RTTs measured within last RTT (in usec) */ 66 u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */ 67 }; 68 69 /* There are several situations when we must "re-start" Vegas: 70 * 71 * o when a connection is established 72 * o after an RTO 73 * o after fast recovery 74 * o when we send a packet and there is no outstanding 75 * unacknowledged data (restarting an idle connection) 76 * 77 * In these circumstances we cannot do a Vegas calculation at the 78 * end of the first RTT, because any calculation we do is using 79 * stale info -- both the saved cwnd and congestion feedback are 80 * stale. 81 * 82 * Instead we must wait until the completion of an RTT during 83 * which we actually receive ACKs. 84 */ 85 static inline void vegas_enable(struct sock *sk) 86 { 87 const struct tcp_sock *tp = tcp_sk(sk); 88 struct vegas *vegas = inet_csk_ca(sk); 89 90 /* Begin taking Vegas samples next time we send something. */ 91 vegas->doing_vegas_now = 1; 92 93 /* Set the beginning of the next send window. */ 94 vegas->beg_snd_nxt = tp->snd_nxt; 95 96 vegas->cntRTT = 0; 97 vegas->minRTT = 0x7fffffff; 98 } 99 100 /* Stop taking Vegas samples for now. */ 101 static inline void vegas_disable(struct sock *sk) 102 { 103 struct vegas *vegas = inet_csk_ca(sk); 104 105 vegas->doing_vegas_now = 0; 106 } 107 108 static void tcp_vegas_init(struct sock *sk) 109 { 110 struct vegas *vegas = inet_csk_ca(sk); 111 112 vegas->baseRTT = 0x7fffffff; 113 vegas_enable(sk); 114 } 115 116 /* Do RTT sampling needed for Vegas. 117 * Basically we: 118 * o min-filter RTT samples from within an RTT to get the current 119 * propagation delay + queuing delay (we are min-filtering to try to 120 * avoid the effects of delayed ACKs) 121 * o min-filter RTT samples from a much longer window (forever for now) 122 * to find the propagation delay (baseRTT) 123 */ 124 static void tcp_vegas_rtt_calc(struct sock *sk, u32 usrtt) 125 { 126 struct vegas *vegas = inet_csk_ca(sk); 127 u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */ 128 129 /* Filter to find propagation delay: */ 130 if (vrtt < vegas->baseRTT) 131 vegas->baseRTT = vrtt; 132 133 /* Find the min RTT during the last RTT to find 134 * the current prop. delay + queuing delay: 135 */ 136 vegas->minRTT = min(vegas->minRTT, vrtt); 137 vegas->cntRTT++; 138 } 139 140 static void tcp_vegas_state(struct sock *sk, u8 ca_state) 141 { 142 143 if (ca_state == TCP_CA_Open) 144 vegas_enable(sk); 145 else 146 vegas_disable(sk); 147 } 148 149 /* 150 * If the connection is idle and we are restarting, 151 * then we don't want to do any Vegas calculations 152 * until we get fresh RTT samples. So when we 153 * restart, we reset our Vegas state to a clean 154 * slate. After we get acks for this flight of 155 * packets, _then_ we can make Vegas calculations 156 * again. 157 */ 158 static void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) 159 { 160 if (event == CA_EVENT_CWND_RESTART || 161 event == CA_EVENT_TX_START) 162 tcp_vegas_init(sk); 163 } 164 165 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, 166 u32 seq_rtt, u32 in_flight, int flag) 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 return tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag); 173 174 /* The key players are v_beg_snd_una and v_beg_snd_nxt. 175 * 176 * These are so named because they represent the approximate values 177 * of snd_una and snd_nxt at the beginning of the current RTT. More 178 * precisely, they represent the amount of data sent during the RTT. 179 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, 180 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding 181 * bytes of data have been ACKed during the course of the RTT, giving 182 * an "actual" rate of: 183 * 184 * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) 185 * 186 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, 187 * because delayed ACKs can cover more than one segment, so they 188 * don't line up nicely with the boundaries of RTTs. 189 * 190 * Another unfortunate fact of life is that delayed ACKs delay the 191 * advance of the left edge of our send window, so that the number 192 * of bytes we send in an RTT is often less than our cwnd will allow. 193 * So we keep track of our cwnd separately, in v_beg_snd_cwnd. 194 */ 195 196 if (after(ack, vegas->beg_snd_nxt)) { 197 /* Do the Vegas once-per-RTT cwnd adjustment. */ 198 u32 old_wnd, old_snd_cwnd; 199 200 201 /* Here old_wnd is essentially the window of data that was 202 * sent during the previous RTT, and has all 203 * been acknowledged in the course of the RTT that ended 204 * with the ACK we just received. Likewise, old_snd_cwnd 205 * is the cwnd during the previous RTT. 206 */ 207 old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / 208 tp->mss_cache; 209 old_snd_cwnd = vegas->beg_snd_cwnd; 210 211 /* Save the extent of the current window so we can use this 212 * at the end of the next RTT. 213 */ 214 vegas->beg_snd_una = vegas->beg_snd_nxt; 215 vegas->beg_snd_nxt = tp->snd_nxt; 216 vegas->beg_snd_cwnd = tp->snd_cwnd; 217 218 /* Take into account the current RTT sample too, to 219 * decrease the impact of delayed acks. This double counts 220 * this sample since we count it for the next window as well, 221 * but that's not too awful, since we're taking the min, 222 * rather than averaging. 223 */ 224 tcp_vegas_rtt_calc(sk, seq_rtt * 1000); 225 226 /* We do the Vegas calculations only if we got enough RTT 227 * samples that we can be reasonably sure that we got 228 * at least one RTT sample that wasn't from a delayed ACK. 229 * If we only had 2 samples total, 230 * then that means we're getting only 1 ACK per RTT, which 231 * means they're almost certainly delayed ACKs. 232 * If we have 3 samples, we should be OK. 233 */ 234 235 if (vegas->cntRTT <= 2) { 236 /* We don't have enough RTT samples to do the Vegas 237 * calculation, so we'll behave like Reno. 238 */ 239 if (tp->snd_cwnd > tp->snd_ssthresh) 240 tp->snd_cwnd++; 241 } else { 242 u32 rtt, target_cwnd, diff; 243 244 /* We have enough RTT samples, so, using the Vegas 245 * algorithm, we determine if we should increase or 246 * decrease cwnd, and by how much. 247 */ 248 249 /* Pluck out the RTT we are using for the Vegas 250 * calculations. This is the min RTT seen during the 251 * last RTT. Taking the min filters out the effects 252 * of delayed ACKs, at the cost of noticing congestion 253 * a bit later. 254 */ 255 rtt = vegas->minRTT; 256 257 /* Calculate the cwnd we should have, if we weren't 258 * going too fast. 259 * 260 * This is: 261 * (actual rate in segments) * baseRTT 262 * We keep it as a fixed point number with 263 * V_PARAM_SHIFT bits to the right of the binary point. 264 */ 265 target_cwnd = ((old_wnd * vegas->baseRTT) 266 << V_PARAM_SHIFT) / rtt; 267 268 /* Calculate the difference between the window we had, 269 * and the window we would like to have. This quantity 270 * is the "Diff" from the Arizona Vegas papers. 271 * 272 * Again, this is a fixed point number with 273 * V_PARAM_SHIFT bits to the right of the binary 274 * point. 275 */ 276 diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; 277 278 if (tp->snd_cwnd < tp->snd_ssthresh) { 279 /* Slow start. */ 280 if (diff > gamma) { 281 /* Going too fast. Time to slow down 282 * and switch to congestion avoidance. 283 */ 284 tp->snd_ssthresh = 2; 285 286 /* Set cwnd to match the actual rate 287 * exactly: 288 * cwnd = (actual rate) * baseRTT 289 * Then we add 1 because the integer 290 * truncation robs us of full link 291 * utilization. 292 */ 293 tp->snd_cwnd = min(tp->snd_cwnd, 294 (target_cwnd >> 295 V_PARAM_SHIFT)+1); 296 297 } 298 } else { 299 /* Congestion avoidance. */ 300 u32 next_snd_cwnd; 301 302 /* Figure out where we would like cwnd 303 * to be. 304 */ 305 if (diff > beta) { 306 /* The old window was too fast, so 307 * we slow down. 308 */ 309 next_snd_cwnd = old_snd_cwnd - 1; 310 } else if (diff < alpha) { 311 /* We don't have enough extra packets 312 * in the network, so speed up. 313 */ 314 next_snd_cwnd = old_snd_cwnd + 1; 315 } else { 316 /* Sending just as fast as we 317 * should be. 318 */ 319 next_snd_cwnd = old_snd_cwnd; 320 } 321 322 /* Adjust cwnd upward or downward, toward the 323 * desired value. 324 */ 325 if (next_snd_cwnd > tp->snd_cwnd) 326 tp->snd_cwnd++; 327 else if (next_snd_cwnd < tp->snd_cwnd) 328 tp->snd_cwnd--; 329 } 330 } 331 332 /* Wipe the slate clean for the next RTT. */ 333 vegas->cntRTT = 0; 334 vegas->minRTT = 0x7fffffff; 335 } 336 337 /* The following code is executed for every ack we receive, 338 * except for conditions checked in should_advance_cwnd() 339 * before the call to tcp_cong_avoid(). Mainly this means that 340 * we only execute this code if the ack actually acked some 341 * data. 342 */ 343 344 /* If we are in slow start, increase our cwnd in response to this ACK. 345 * (If we are not in slow start then we are in congestion avoidance, 346 * and adjust our congestion window only once per RTT. See the code 347 * above.) 348 */ 349 if (tp->snd_cwnd <= tp->snd_ssthresh) 350 tp->snd_cwnd++; 351 352 /* to keep cwnd from growing without bound */ 353 tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp); 354 355 /* Make sure that we are never so timid as to reduce our cwnd below 356 * 2 MSS. 357 * 358 * Going below 2 MSS would risk huge delayed ACKs from our receiver. 359 */ 360 tp->snd_cwnd = max(tp->snd_cwnd, 2U); 361 } 362 363 /* Extract info for Tcp socket info provided via netlink. */ 364 static void tcp_vegas_get_info(struct sock *sk, u32 ext, 365 struct sk_buff *skb) 366 { 367 const struct vegas *ca = inet_csk_ca(sk); 368 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 369 struct tcpvegas_info *info; 370 371 info = RTA_DATA(__RTA_PUT(skb, INET_DIAG_VEGASINFO, 372 sizeof(*info))); 373 374 info->tcpv_enabled = ca->doing_vegas_now; 375 info->tcpv_rttcnt = ca->cntRTT; 376 info->tcpv_rtt = ca->baseRTT; 377 info->tcpv_minrtt = ca->minRTT; 378 rtattr_failure: ; 379 } 380 } 381 382 static struct tcp_congestion_ops tcp_vegas = { 383 .init = tcp_vegas_init, 384 .ssthresh = tcp_reno_ssthresh, 385 .cong_avoid = tcp_vegas_cong_avoid, 386 .min_cwnd = tcp_reno_min_cwnd, 387 .rtt_sample = tcp_vegas_rtt_calc, 388 .set_state = tcp_vegas_state, 389 .cwnd_event = tcp_vegas_cwnd_event, 390 .get_info = tcp_vegas_get_info, 391 392 .owner = THIS_MODULE, 393 .name = "vegas", 394 }; 395 396 static int __init tcp_vegas_register(void) 397 { 398 BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); 399 tcp_register_congestion_control(&tcp_vegas); 400 return 0; 401 } 402 403 static void __exit tcp_vegas_unregister(void) 404 { 405 tcp_unregister_congestion_control(&tcp_vegas); 406 } 407 408 module_init(tcp_vegas_register); 409 module_exit(tcp_vegas_unregister); 410 411 MODULE_AUTHOR("Stephen Hemminger"); 412 MODULE_LICENSE("GPL"); 413 MODULE_DESCRIPTION("TCP Vegas"); 414