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 /* Default values of the Vegas variables, in fixed-point representation 44 * with V_PARAM_SHIFT bits to the right of the binary point. 45 */ 46 #define V_PARAM_SHIFT 1 47 static int alpha = 2<<V_PARAM_SHIFT; 48 static int beta = 4<<V_PARAM_SHIFT; 49 static int gamma = 1<<V_PARAM_SHIFT; 50 51 module_param(alpha, int, 0644); 52 MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)"); 53 module_param(beta, int, 0644); 54 MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)"); 55 module_param(gamma, int, 0644); 56 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); 57 58 59 /* There are several situations when we must "re-start" Vegas: 60 * 61 * o when a connection is established 62 * o after an RTO 63 * o after fast recovery 64 * o when we send a packet and there is no outstanding 65 * unacknowledged data (restarting an idle connection) 66 * 67 * In these circumstances we cannot do a Vegas calculation at the 68 * end of the first RTT, because any calculation we do is using 69 * stale info -- both the saved cwnd and congestion feedback are 70 * stale. 71 * 72 * Instead we must wait until the completion of an RTT during 73 * which we actually receive ACKs. 74 */ 75 static void vegas_enable(struct sock *sk) 76 { 77 const struct tcp_sock *tp = tcp_sk(sk); 78 struct vegas *vegas = inet_csk_ca(sk); 79 80 /* Begin taking Vegas samples next time we send something. */ 81 vegas->doing_vegas_now = 1; 82 83 /* Set the beginning of the next send window. */ 84 vegas->beg_snd_nxt = tp->snd_nxt; 85 86 vegas->cntRTT = 0; 87 vegas->minRTT = 0x7fffffff; 88 } 89 90 /* Stop taking Vegas samples for now. */ 91 static inline void vegas_disable(struct sock *sk) 92 { 93 struct vegas *vegas = inet_csk_ca(sk); 94 95 vegas->doing_vegas_now = 0; 96 } 97 98 void tcp_vegas_init(struct sock *sk) 99 { 100 struct vegas *vegas = inet_csk_ca(sk); 101 102 vegas->baseRTT = 0x7fffffff; 103 vegas_enable(sk); 104 } 105 EXPORT_SYMBOL_GPL(tcp_vegas_init); 106 107 /* Do RTT sampling needed for Vegas. 108 * Basically we: 109 * o min-filter RTT samples from within an RTT to get the current 110 * propagation delay + queuing delay (we are min-filtering to try to 111 * avoid the effects of delayed ACKs) 112 * o min-filter RTT samples from a much longer window (forever for now) 113 * to find the propagation delay (baseRTT) 114 */ 115 void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us) 116 { 117 struct vegas *vegas = inet_csk_ca(sk); 118 u32 vrtt; 119 120 if (rtt_us < 0) 121 return; 122 123 /* Never allow zero rtt or baseRTT */ 124 vrtt = rtt_us + 1; 125 126 /* Filter to find propagation delay: */ 127 if (vrtt < vegas->baseRTT) 128 vegas->baseRTT = vrtt; 129 130 /* Find the min RTT during the last RTT to find 131 * the current prop. delay + queuing delay: 132 */ 133 vegas->minRTT = min(vegas->minRTT, vrtt); 134 vegas->cntRTT++; 135 } 136 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked); 137 138 void tcp_vegas_state(struct sock *sk, u8 ca_state) 139 { 140 141 if (ca_state == TCP_CA_Open) 142 vegas_enable(sk); 143 else 144 vegas_disable(sk); 145 } 146 EXPORT_SYMBOL_GPL(tcp_vegas_state); 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 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) 158 { 159 if (event == CA_EVENT_CWND_RESTART || 160 event == CA_EVENT_TX_START) 161 tcp_vegas_init(sk); 162 } 163 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event); 164 165 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight) 166 { 167 struct tcp_sock *tp = tcp_sk(sk); 168 struct vegas *vegas = inet_csk_ca(sk); 169 170 if (!vegas->doing_vegas_now) { 171 tcp_reno_cong_avoid(sk, ack, in_flight); 172 return; 173 } 174 175 /* The key players are v_beg_snd_una and v_beg_snd_nxt. 176 * 177 * These are so named because they represent the approximate values 178 * of snd_una and snd_nxt at the beginning of the current RTT. More 179 * precisely, they represent the amount of data sent during the RTT. 180 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, 181 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding 182 * bytes of data have been ACKed during the course of the RTT, giving 183 * an "actual" rate of: 184 * 185 * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) 186 * 187 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, 188 * because delayed ACKs can cover more than one segment, so they 189 * don't line up nicely with the boundaries of RTTs. 190 * 191 * Another unfortunate fact of life is that delayed ACKs delay the 192 * advance of the left edge of our send window, so that the number 193 * of bytes we send in an RTT is often less than our cwnd will allow. 194 * So we keep track of our cwnd separately, in v_beg_snd_cwnd. 195 */ 196 197 if (after(ack, vegas->beg_snd_nxt)) { 198 /* Do the Vegas once-per-RTT cwnd adjustment. */ 199 u32 old_wnd, old_snd_cwnd; 200 201 202 /* Here old_wnd is essentially the window of data that was 203 * sent during the previous RTT, and has all 204 * been acknowledged in the course of the RTT that ended 205 * with the ACK we just received. Likewise, old_snd_cwnd 206 * is the cwnd during the previous RTT. 207 */ 208 old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / 209 tp->mss_cache; 210 old_snd_cwnd = vegas->beg_snd_cwnd; 211 212 /* Save the extent of the current window so we can use this 213 * at the end of the next RTT. 214 */ 215 vegas->beg_snd_una = vegas->beg_snd_nxt; 216 vegas->beg_snd_nxt = tp->snd_nxt; 217 vegas->beg_snd_cwnd = tp->snd_cwnd; 218 219 /* We do the Vegas calculations only if we got enough RTT 220 * samples that we can be reasonably sure that we got 221 * at least one RTT sample that wasn't from a delayed ACK. 222 * If we only had 2 samples total, 223 * then that means we're getting only 1 ACK per RTT, which 224 * means they're almost certainly delayed ACKs. 225 * If we have 3 samples, we should be OK. 226 */ 227 228 if (vegas->cntRTT <= 2) { 229 /* We don't have enough RTT samples to do the Vegas 230 * calculation, so we'll behave like Reno. 231 */ 232 tcp_reno_cong_avoid(sk, ack, in_flight); 233 } else { 234 u32 rtt, diff; 235 u64 target_cwnd; 236 237 /* We have enough RTT samples, so, using the Vegas 238 * algorithm, we determine if we should increase or 239 * decrease cwnd, and by how much. 240 */ 241 242 /* Pluck out the RTT we are using for the Vegas 243 * calculations. This is the min RTT seen during the 244 * last RTT. Taking the min filters out the effects 245 * of delayed ACKs, at the cost of noticing congestion 246 * a bit later. 247 */ 248 rtt = vegas->minRTT; 249 250 /* Calculate the cwnd we should have, if we weren't 251 * going too fast. 252 * 253 * This is: 254 * (actual rate in segments) * baseRTT 255 * We keep it as a fixed point number with 256 * V_PARAM_SHIFT bits to the right of the binary point. 257 */ 258 target_cwnd = ((u64)old_wnd * vegas->baseRTT); 259 target_cwnd <<= V_PARAM_SHIFT; 260 do_div(target_cwnd, rtt); 261 262 /* Calculate the difference between the window we had, 263 * and the window we would like to have. This quantity 264 * is the "Diff" from the Arizona Vegas papers. 265 * 266 * Again, this is a fixed point number with 267 * V_PARAM_SHIFT bits to the right of the binary 268 * point. 269 */ 270 diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; 271 272 if (diff > gamma && tp->snd_ssthresh > 2 ) { 273 /* Going too fast. Time to slow down 274 * and switch to congestion avoidance. 275 */ 276 tp->snd_ssthresh = 2; 277 278 /* Set cwnd to match the actual rate 279 * exactly: 280 * cwnd = (actual rate) * baseRTT 281 * Then we add 1 because the integer 282 * truncation robs us of full link 283 * utilization. 284 */ 285 tp->snd_cwnd = min(tp->snd_cwnd, 286 ((u32)target_cwnd >> 287 V_PARAM_SHIFT)+1); 288 289 } else if (tp->snd_cwnd <= tp->snd_ssthresh) { 290 /* Slow start. */ 291 tcp_slow_start(tp); 292 } else { 293 /* Congestion avoidance. */ 294 u32 next_snd_cwnd; 295 296 /* Figure out where we would like cwnd 297 * to be. 298 */ 299 if (diff > beta) { 300 /* The old window was too fast, so 301 * we slow down. 302 */ 303 next_snd_cwnd = old_snd_cwnd - 1; 304 } else if (diff < alpha) { 305 /* We don't have enough extra packets 306 * in the network, so speed up. 307 */ 308 next_snd_cwnd = old_snd_cwnd + 1; 309 } else { 310 /* Sending just as fast as we 311 * should be. 312 */ 313 next_snd_cwnd = old_snd_cwnd; 314 } 315 316 /* Adjust cwnd upward or downward, toward the 317 * desired value. 318 */ 319 if (next_snd_cwnd > tp->snd_cwnd) 320 tp->snd_cwnd++; 321 else if (next_snd_cwnd < tp->snd_cwnd) 322 tp->snd_cwnd--; 323 } 324 325 if (tp->snd_cwnd < 2) 326 tp->snd_cwnd = 2; 327 else if (tp->snd_cwnd > tp->snd_cwnd_clamp) 328 tp->snd_cwnd = tp->snd_cwnd_clamp; 329 } 330 331 /* Wipe the slate clean for the next RTT. */ 332 vegas->cntRTT = 0; 333 vegas->minRTT = 0x7fffffff; 334 } 335 /* Use normal slow start */ 336 else if (tp->snd_cwnd <= tp->snd_ssthresh) 337 tcp_slow_start(tp); 338 339 } 340 341 /* Extract info for Tcp socket info provided via netlink. */ 342 void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb) 343 { 344 const struct vegas *ca = inet_csk_ca(sk); 345 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 346 struct tcpvegas_info info = { 347 .tcpv_enabled = ca->doing_vegas_now, 348 .tcpv_rttcnt = ca->cntRTT, 349 .tcpv_rtt = ca->baseRTT, 350 .tcpv_minrtt = ca->minRTT, 351 }; 352 353 nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info); 354 } 355 } 356 EXPORT_SYMBOL_GPL(tcp_vegas_get_info); 357 358 static struct tcp_congestion_ops tcp_vegas = { 359 .flags = TCP_CONG_RTT_STAMP, 360 .init = tcp_vegas_init, 361 .ssthresh = tcp_reno_ssthresh, 362 .cong_avoid = tcp_vegas_cong_avoid, 363 .min_cwnd = tcp_reno_min_cwnd, 364 .pkts_acked = tcp_vegas_pkts_acked, 365 .set_state = tcp_vegas_state, 366 .cwnd_event = tcp_vegas_cwnd_event, 367 .get_info = tcp_vegas_get_info, 368 369 .owner = THIS_MODULE, 370 .name = "vegas", 371 }; 372 373 static int __init tcp_vegas_register(void) 374 { 375 BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); 376 tcp_register_congestion_control(&tcp_vegas); 377 return 0; 378 } 379 380 static void __exit tcp_vegas_unregister(void) 381 { 382 tcp_unregister_congestion_control(&tcp_vegas); 383 } 384 385 module_init(tcp_vegas_register); 386 module_exit(tcp_vegas_unregister); 387 388 MODULE_AUTHOR("Stephen Hemminger"); 389 MODULE_LICENSE("GPL"); 390 MODULE_DESCRIPTION("TCP Vegas"); 391