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 /* Default values of the Vegas variables, in fixed-point representation 42 * with V_PARAM_SHIFT bits to the right of the binary point. 43 */ 44 #define V_PARAM_SHIFT 1 45 static int alpha = 1<<V_PARAM_SHIFT; 46 static int beta = 3<<V_PARAM_SHIFT; 47 static int gamma = 1<<V_PARAM_SHIFT; 48 49 module_param(alpha, int, 0644); 50 MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)"); 51 module_param(beta, int, 0644); 52 MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)"); 53 module_param(gamma, int, 0644); 54 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); 55 56 57 /* Vegas variables */ 58 struct vegas { 59 u32 beg_snd_nxt; /* right edge during last RTT */ 60 u32 beg_snd_una; /* left edge during last RTT */ 61 u32 beg_snd_cwnd; /* saves the size of the cwnd */ 62 u8 doing_vegas_now;/* if true, do vegas for this RTT */ 63 u16 cntRTT; /* # of RTTs measured within last RTT */ 64 u32 minRTT; /* min of RTTs measured within last RTT (in usec) */ 65 u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */ 66 }; 67 68 /* There are several situations when we must "re-start" Vegas: 69 * 70 * o when a connection is established 71 * o after an RTO 72 * o after fast recovery 73 * o when we send a packet and there is no outstanding 74 * unacknowledged data (restarting an idle connection) 75 * 76 * In these circumstances we cannot do a Vegas calculation at the 77 * end of the first RTT, because any calculation we do is using 78 * stale info -- both the saved cwnd and congestion feedback are 79 * stale. 80 * 81 * Instead we must wait until the completion of an RTT during 82 * which we actually receive ACKs. 83 */ 84 static inline void vegas_enable(struct sock *sk) 85 { 86 const struct tcp_sock *tp = tcp_sk(sk); 87 struct vegas *vegas = inet_csk_ca(sk); 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 sock *sk) 101 { 102 struct vegas *vegas = inet_csk_ca(sk); 103 104 vegas->doing_vegas_now = 0; 105 } 106 107 static void tcp_vegas_init(struct sock *sk) 108 { 109 struct vegas *vegas = inet_csk_ca(sk); 110 111 vegas->baseRTT = 0x7fffffff; 112 vegas_enable(sk); 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 sock *sk, u32 usrtt) 124 { 125 struct vegas *vegas = inet_csk_ca(sk); 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 sock *sk, u8 ca_state) 140 { 141 142 if (ca_state == TCP_CA_Open) 143 vegas_enable(sk); 144 else 145 vegas_disable(sk); 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 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 } 163 164 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, 165 u32 seq_rtt, u32 in_flight, int flag) 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 return tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag); 172 173 /* The key players are v_beg_snd_una and v_beg_snd_nxt. 174 * 175 * These are so named because they represent the approximate values 176 * of snd_una and snd_nxt at the beginning of the current RTT. More 177 * precisely, they represent the amount of data sent during the RTT. 178 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, 179 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding 180 * bytes of data have been ACKed during the course of the RTT, giving 181 * an "actual" rate of: 182 * 183 * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) 184 * 185 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, 186 * because delayed ACKs can cover more than one segment, so they 187 * don't line up nicely with the boundaries of RTTs. 188 * 189 * Another unfortunate fact of life is that delayed ACKs delay the 190 * advance of the left edge of our send window, so that the number 191 * of bytes we send in an RTT is often less than our cwnd will allow. 192 * So we keep track of our cwnd separately, in v_beg_snd_cwnd. 193 */ 194 195 if (after(ack, vegas->beg_snd_nxt)) { 196 /* Do the Vegas once-per-RTT cwnd adjustment. */ 197 u32 old_wnd, old_snd_cwnd; 198 199 200 /* Here old_wnd is essentially the window of data that was 201 * sent during the previous RTT, and has all 202 * been acknowledged in the course of the RTT that ended 203 * with the ACK we just received. Likewise, old_snd_cwnd 204 * is the cwnd during the previous RTT. 205 */ 206 old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / 207 tp->mss_cache; 208 old_snd_cwnd = vegas->beg_snd_cwnd; 209 210 /* Save the extent of the current window so we can use this 211 * at the end of the next RTT. 212 */ 213 vegas->beg_snd_una = vegas->beg_snd_nxt; 214 vegas->beg_snd_nxt = tp->snd_nxt; 215 vegas->beg_snd_cwnd = tp->snd_cwnd; 216 217 /* We do the Vegas calculations only if we got enough RTT 218 * samples that we can be reasonably sure that we got 219 * at least one RTT sample that wasn't from a delayed ACK. 220 * If we only had 2 samples total, 221 * then that means we're getting only 1 ACK per RTT, which 222 * means they're almost certainly delayed ACKs. 223 * If we have 3 samples, we should be OK. 224 */ 225 226 if (vegas->cntRTT <= 2) { 227 /* We don't have enough RTT samples to do the Vegas 228 * calculation, so we'll behave like Reno. 229 */ 230 tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag); 231 } else { 232 u32 rtt, target_cwnd, diff; 233 234 /* We have enough RTT samples, so, using the Vegas 235 * algorithm, we determine if we should increase or 236 * decrease cwnd, and by how much. 237 */ 238 239 /* Pluck out the RTT we are using for the Vegas 240 * calculations. This is the min RTT seen during the 241 * last RTT. Taking the min filters out the effects 242 * of delayed ACKs, at the cost of noticing congestion 243 * a bit later. 244 */ 245 rtt = vegas->minRTT; 246 247 /* Calculate the cwnd we should have, if we weren't 248 * going too fast. 249 * 250 * This is: 251 * (actual rate in segments) * baseRTT 252 * We keep it as a fixed point number with 253 * V_PARAM_SHIFT bits to the right of the binary point. 254 */ 255 target_cwnd = ((old_wnd * vegas->baseRTT) 256 << V_PARAM_SHIFT) / rtt; 257 258 /* Calculate the difference between the window we had, 259 * and the window we would like to have. This quantity 260 * is the "Diff" from the Arizona Vegas papers. 261 * 262 * Again, this is a fixed point number with 263 * V_PARAM_SHIFT bits to the right of the binary 264 * point. 265 */ 266 diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; 267 268 if (tp->snd_cwnd <= tp->snd_ssthresh) { 269 /* Slow start. */ 270 if (diff > gamma) { 271 /* Going too fast. Time to slow down 272 * and switch to congestion avoidance. 273 */ 274 tp->snd_ssthresh = 2; 275 276 /* Set cwnd to match the actual rate 277 * exactly: 278 * cwnd = (actual rate) * baseRTT 279 * Then we add 1 because the integer 280 * truncation robs us of full link 281 * utilization. 282 */ 283 tp->snd_cwnd = min(tp->snd_cwnd, 284 (target_cwnd >> 285 V_PARAM_SHIFT)+1); 286 287 } 288 tcp_slow_start(tp); 289 } else { 290 /* Congestion avoidance. */ 291 u32 next_snd_cwnd; 292 293 /* Figure out where we would like cwnd 294 * to be. 295 */ 296 if (diff > beta) { 297 /* The old window was too fast, so 298 * we slow down. 299 */ 300 next_snd_cwnd = old_snd_cwnd - 1; 301 } else if (diff < alpha) { 302 /* We don't have enough extra packets 303 * in the network, so speed up. 304 */ 305 next_snd_cwnd = old_snd_cwnd + 1; 306 } else { 307 /* Sending just as fast as we 308 * should be. 309 */ 310 next_snd_cwnd = old_snd_cwnd; 311 } 312 313 /* Adjust cwnd upward or downward, toward the 314 * desired value. 315 */ 316 if (next_snd_cwnd > tp->snd_cwnd) 317 tp->snd_cwnd++; 318 else if (next_snd_cwnd < tp->snd_cwnd) 319 tp->snd_cwnd--; 320 } 321 322 if (tp->snd_cwnd < 2) 323 tp->snd_cwnd = 2; 324 else if (tp->snd_cwnd > tp->snd_cwnd_clamp) 325 tp->snd_cwnd = tp->snd_cwnd_clamp; 326 } 327 328 /* Wipe the slate clean for the next RTT. */ 329 vegas->cntRTT = 0; 330 vegas->minRTT = 0x7fffffff; 331 } 332 /* Use normal slow start */ 333 else if (tp->snd_cwnd <= tp->snd_ssthresh) 334 tcp_slow_start(tp); 335 336 } 337 338 /* Extract info for Tcp socket info provided via netlink. */ 339 static void tcp_vegas_get_info(struct sock *sk, u32 ext, 340 struct sk_buff *skb) 341 { 342 const struct vegas *ca = inet_csk_ca(sk); 343 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { 344 struct tcpvegas_info *info; 345 346 info = RTA_DATA(__RTA_PUT(skb, INET_DIAG_VEGASINFO, 347 sizeof(*info))); 348 349 info->tcpv_enabled = ca->doing_vegas_now; 350 info->tcpv_rttcnt = ca->cntRTT; 351 info->tcpv_rtt = ca->baseRTT; 352 info->tcpv_minrtt = ca->minRTT; 353 rtattr_failure: ; 354 } 355 } 356 357 static struct tcp_congestion_ops tcp_vegas = { 358 .init = tcp_vegas_init, 359 .ssthresh = tcp_reno_ssthresh, 360 .cong_avoid = tcp_vegas_cong_avoid, 361 .min_cwnd = tcp_reno_min_cwnd, 362 .rtt_sample = tcp_vegas_rtt_calc, 363 .set_state = tcp_vegas_state, 364 .cwnd_event = tcp_vegas_cwnd_event, 365 .get_info = tcp_vegas_get_info, 366 367 .owner = THIS_MODULE, 368 .name = "vegas", 369 }; 370 371 static int __init tcp_vegas_register(void) 372 { 373 BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); 374 tcp_register_congestion_control(&tcp_vegas); 375 return 0; 376 } 377 378 static void __exit tcp_vegas_unregister(void) 379 { 380 tcp_unregister_congestion_control(&tcp_vegas); 381 } 382 383 module_init(tcp_vegas_register); 384 module_exit(tcp_vegas_unregister); 385 386 MODULE_AUTHOR("Stephen Hemminger"); 387 MODULE_LICENSE("GPL"); 388 MODULE_DESCRIPTION("TCP Vegas"); 389