net/ipv4/tcp_vegas.c

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