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/tcp_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 tcp_sock *tp) 86 { 87 struct vegas *vegas = tcp_ca(tp); 88 89 /* Begin taking Vegas samples next time we send something. */ 90 vegas->doing_vegas_now = 1; 91 92 /* Set the beginning of the next send window. */ 93 vegas->beg_snd_nxt = tp->snd_nxt; 94 95 vegas->cntRTT = 0; 96 vegas->minRTT = 0x7fffffff; 97 } 98 99 /* Stop taking Vegas samples for now. */ 100 static inline void vegas_disable(struct tcp_sock *tp) 101 { 102 struct vegas *vegas = tcp_ca(tp); 103 104 vegas->doing_vegas_now = 0; 105 } 106 107 static void tcp_vegas_init(struct tcp_sock *tp) 108 { 109 struct vegas *vegas = tcp_ca(tp); 110 111 vegas->baseRTT = 0x7fffffff; 112 vegas_enable(tp); 113 } 114 115 /* Do RTT sampling needed for Vegas. 116 * Basically we: 117 * o min-filter RTT samples from within an RTT to get the current 118 * propagation delay + queuing delay (we are min-filtering to try to 119 * avoid the effects of delayed ACKs) 120 * o min-filter RTT samples from a much longer window (forever for now) 121 * to find the propagation delay (baseRTT) 122 */ 123 static void tcp_vegas_rtt_calc(struct tcp_sock *tp, u32 usrtt) 124 { 125 struct vegas *vegas = tcp_ca(tp); 126 u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */ 127 128 /* Filter to find propagation delay: */ 129 if (vrtt < vegas->baseRTT) 130 vegas->baseRTT = vrtt; 131 132 /* Find the min RTT during the last RTT to find 133 * the current prop. delay + queuing delay: 134 */ 135 vegas->minRTT = min(vegas->minRTT, vrtt); 136 vegas->cntRTT++; 137 } 138 139 static void tcp_vegas_state(struct tcp_sock *tp, u8 ca_state) 140 { 141 142 if (ca_state == TCP_CA_Open) 143 vegas_enable(tp); 144 else 145 vegas_disable(tp); 146 } 147 148 /* 149 * If the connection is idle and we are restarting, 150 * then we don't want to do any Vegas calculations 151 * until we get fresh RTT samples. So when we 152 * restart, we reset our Vegas state to a clean 153 * slate. After we get acks for this flight of 154 * packets, _then_ we can make Vegas calculations 155 * again. 156 */ 157 static void tcp_vegas_cwnd_event(struct tcp_sock *tp, enum tcp_ca_event event) 158 { 159 if (event == CA_EVENT_CWND_RESTART || 160 event == CA_EVENT_TX_START) 161 tcp_vegas_init(tp); 162 } 163 164 static void tcp_vegas_cong_avoid(struct tcp_sock *tp, u32 ack, 165 u32 seq_rtt, u32 in_flight, int flag) 166 { 167 struct vegas *vegas = tcp_ca(tp); 168 169 if (!vegas->doing_vegas_now) 170 return tcp_reno_cong_avoid(tp, ack, seq_rtt, in_flight, flag); 171 172 /* The key players are v_beg_snd_una and v_beg_snd_nxt. 173 * 174 * These are so named because they represent the approximate values 175 * of snd_una and snd_nxt at the beginning of the current RTT. More 176 * precisely, they represent the amount of data sent during the RTT. 177 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, 178 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding 179 * bytes of data have been ACKed during the course of the RTT, giving 180 * an "actual" rate of: 181 * 182 * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) 183 * 184 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, 185 * because delayed ACKs can cover more than one segment, so they 186 * don't line up nicely with the boundaries of RTTs. 187 * 188 * Another unfortunate fact of life is that delayed ACKs delay the 189 * advance of the left edge of our send window, so that the number 190 * of bytes we send in an RTT is often less than our cwnd will allow. 191 * So we keep track of our cwnd separately, in v_beg_snd_cwnd. 192 */ 193 194 if (after(ack, vegas->beg_snd_nxt)) { 195 /* Do the Vegas once-per-RTT cwnd adjustment. */ 196 u32 old_wnd, old_snd_cwnd; 197 198 199 /* Here old_wnd is essentially the window of data that was 200 * sent during the previous RTT, and has all 201 * been acknowledged in the course of the RTT that ended 202 * with the ACK we just received. Likewise, old_snd_cwnd 203 * is the cwnd during the previous RTT. 204 */ 205 old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / 206 tp->mss_cache; 207 old_snd_cwnd = vegas->beg_snd_cwnd; 208 209 /* Save the extent of the current window so we can use this 210 * at the end of the next RTT. 211 */ 212 vegas->beg_snd_una = vegas->beg_snd_nxt; 213 vegas->beg_snd_nxt = tp->snd_nxt; 214 vegas->beg_snd_cwnd = tp->snd_cwnd; 215 216 /* Take into account the current RTT sample too, to 217 * decrease the impact of delayed acks. This double counts 218 * this sample since we count it for the next window as well, 219 * but that's not too awful, since we're taking the min, 220 * rather than averaging. 221 */ 222 tcp_vegas_rtt_calc(tp, seq_rtt*1000); 223 224 /* We do the Vegas calculations only if we got enough RTT 225 * samples that we can be reasonably sure that we got 226 * at least one RTT sample that wasn't from a delayed ACK. 227 * If we only had 2 samples total, 228 * then that means we're getting only 1 ACK per RTT, which 229 * means they're almost certainly delayed ACKs. 230 * If we have 3 samples, we should be OK. 231 */ 232 233 if (vegas->cntRTT <= 2) { 234 /* We don't have enough RTT samples to do the Vegas 235 * calculation, so we'll behave like Reno. 236 */ 237 if (tp->snd_cwnd > tp->snd_ssthresh) 238 tp->snd_cwnd++; 239 } else { 240 u32 rtt, target_cwnd, diff; 241 242 /* We have enough RTT samples, so, using the Vegas 243 * algorithm, we determine if we should increase or 244 * decrease cwnd, and by how much. 245 */ 246 247 /* Pluck out the RTT we are using for the Vegas 248 * calculations. This is the min RTT seen during the 249 * last RTT. Taking the min filters out the effects 250 * of delayed ACKs, at the cost of noticing congestion 251 * a bit later. 252 */ 253 rtt = vegas->minRTT; 254 255 /* Calculate the cwnd we should have, if we weren't 256 * going too fast. 257 * 258 * This is: 259 * (actual rate in segments) * baseRTT 260 * We keep it as a fixed point number with 261 * V_PARAM_SHIFT bits to the right of the binary point. 262 */ 263 target_cwnd = ((old_wnd * vegas->baseRTT) 264 << V_PARAM_SHIFT) / rtt; 265 266 /* Calculate the difference between the window we had, 267 * and the window we would like to have. This quantity 268 * is the "Diff" from the Arizona Vegas papers. 269 * 270 * Again, this is a fixed point number with 271 * V_PARAM_SHIFT bits to the right of the binary 272 * point. 273 */ 274 diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; 275 276 if (tp->snd_cwnd < tp->snd_ssthresh) { 277 /* Slow start. */ 278 if (diff > gamma) { 279 /* Going too fast. Time to slow down 280 * and switch to congestion avoidance. 281 */ 282 tp->snd_ssthresh = 2; 283 284 /* Set cwnd to match the actual rate 285 * exactly: 286 * cwnd = (actual rate) * baseRTT 287 * Then we add 1 because the integer 288 * truncation robs us of full link 289 * utilization. 290 */ 291 tp->snd_cwnd = min(tp->snd_cwnd, 292 (target_cwnd >> 293 V_PARAM_SHIFT)+1); 294 295 } 296 } else { 297 /* Congestion avoidance. */ 298 u32 next_snd_cwnd; 299 300 /* Figure out where we would like cwnd 301 * to be. 302 */ 303 if (diff > beta) { 304 /* The old window was too fast, so 305 * we slow down. 306 */ 307 next_snd_cwnd = old_snd_cwnd - 1; 308 } else if (diff < alpha) { 309 /* We don't have enough extra packets 310 * in the network, so speed up. 311 */ 312 next_snd_cwnd = old_snd_cwnd + 1; 313 } else { 314 /* Sending just as fast as we 315 * should be. 316 */ 317 next_snd_cwnd = old_snd_cwnd; 318 } 319 320 /* Adjust cwnd upward or downward, toward the 321 * desired value. 322 */ 323 if (next_snd_cwnd > tp->snd_cwnd) 324 tp->snd_cwnd++; 325 else if (next_snd_cwnd < tp->snd_cwnd) 326 tp->snd_cwnd--; 327 } 328 } 329 330 /* Wipe the slate clean for the next RTT. */ 331 vegas->cntRTT = 0; 332 vegas->minRTT = 0x7fffffff; 333 } 334 335 /* The following code is executed for every ack we receive, 336 * except for conditions checked in should_advance_cwnd() 337 * before the call to tcp_cong_avoid(). Mainly this means that 338 * we only execute this code if the ack actually acked some 339 * data. 340 */ 341 342 /* If we are in slow start, increase our cwnd in response to this ACK. 343 * (If we are not in slow start then we are in congestion avoidance, 344 * and adjust our congestion window only once per RTT. See the code 345 * above.) 346 */ 347 if (tp->snd_cwnd <= tp->snd_ssthresh) 348 tp->snd_cwnd++; 349 350 /* to keep cwnd from growing without bound */ 351 tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp); 352 353 /* Make sure that we are never so timid as to reduce our cwnd below 354 * 2 MSS. 355 * 356 * Going below 2 MSS would risk huge delayed ACKs from our receiver. 357 */ 358 tp->snd_cwnd = max(tp->snd_cwnd, 2U); 359 } 360 361 /* Extract info for Tcp socket info provided via netlink. */ 362 static void tcp_vegas_get_info(struct tcp_sock *tp, u32 ext, 363 struct sk_buff *skb) 364 { 365 const struct vegas *ca = tcp_ca(tp); 366 if (ext & (1<<(TCPDIAG_VEGASINFO-1))) { 367 struct tcpvegas_info *info; 368 369 info = RTA_DATA(__RTA_PUT(skb, TCPDIAG_VEGASINFO, 370 sizeof(*info))); 371 372 info->tcpv_enabled = ca->doing_vegas_now; 373 info->tcpv_rttcnt = ca->cntRTT; 374 info->tcpv_rtt = ca->baseRTT; 375 info->tcpv_minrtt = ca->minRTT; 376 rtattr_failure: ; 377 } 378 } 379 380 static struct tcp_congestion_ops tcp_vegas = { 381 .init = tcp_vegas_init, 382 .ssthresh = tcp_reno_ssthresh, 383 .cong_avoid = tcp_vegas_cong_avoid, 384 .min_cwnd = tcp_reno_min_cwnd, 385 .rtt_sample = tcp_vegas_rtt_calc, 386 .set_state = tcp_vegas_state, 387 .cwnd_event = tcp_vegas_cwnd_event, 388 .get_info = tcp_vegas_get_info, 389 390 .owner = THIS_MODULE, 391 .name = "vegas", 392 }; 393 394 static int __init tcp_vegas_register(void) 395 { 396 BUG_ON(sizeof(struct vegas) > TCP_CA_PRIV_SIZE); 397 tcp_register_congestion_control(&tcp_vegas); 398 return 0; 399 } 400 401 static void __exit tcp_vegas_unregister(void) 402 { 403 tcp_unregister_congestion_control(&tcp_vegas); 404 } 405 406 module_init(tcp_vegas_register); 407 module_exit(tcp_vegas_unregister); 408 409 MODULE_AUTHOR("Stephen Hemminger"); 410 MODULE_LICENSE("GPL"); 411 MODULE_DESCRIPTION("TCP Vegas"); 412