net/ipv4/tcp_cubic.c

   1 /*
   2  * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
   3  * Home page:
   4  *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
   5  * This is from the implementation of CUBIC TCP in
   6  * Sangtae Ha, Injong Rhee and Lisong Xu,
   7  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
   8  *  in ACM SIGOPS Operating System Review, July 2008.
   9  * Available from:
  10  *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
  11  *
  12  * CUBIC integrates a new slow start algorithm, called HyStart.
  13  * The details of HyStart are presented in
  14  *  Sangtae Ha and Injong Rhee,
  15  *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
  16  * Available from:
  17  *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
  18  *
  19  * All testing results are available from:
  20  * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
  21  *
  22  * Unless CUBIC is enabled and congestion window is large
  23  * this behaves the same as the original Reno.
  24  */
  25
  26 #include <linux/mm.h>
  27 #include <linux/module.h>
  28 #include <linux/math64.h>
  29 #include <net/tcp.h>
  30
  31 #define BICTCP_BETA_SCALE    1024       /* Scale factor beta calculation
  32                                          * max_cwnd = snd_cwnd * beta
  33                                          */
  34 #define BICTCP_HZ               10      /* BIC HZ 2^10 = 1024 */
  35
  36 /* Two methods of hybrid slow start */
  37 #define HYSTART_ACK_TRAIN       0x1
  38 #define HYSTART_DELAY           0x2
  39
  40 /* Number of delay samples for detecting the increase of delay */
  41 #define HYSTART_MIN_SAMPLES     8
  42 #define HYSTART_DELAY_MIN       (4U<<3)
  43 #define HYSTART_DELAY_MAX       (16U<<3)
  44 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
  45
  46 static int fast_convergence __read_mostly = 1;
  47 static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
  48 static int initial_ssthresh __read_mostly;
  49 static int bic_scale __read_mostly = 41;
  50 static int tcp_friendliness __read_mostly = 1;
  51
  52 static int hystart __read_mostly = 1;
  53 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
  54 static int hystart_low_window __read_mostly = 16;
  55 static int hystart_ack_delta __read_mostly = 2;
  56
  57 static u32 cube_rtt_scale __read_mostly;
  58 static u32 beta_scale __read_mostly;
  59 static u64 cube_factor __read_mostly;
  60
  61 /* Note parameters that are used for precomputing scale factors are read-only */
  62 module_param(fast_convergence, int, 0644);
  63 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  64 module_param(beta, int, 0644);
  65 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  66 module_param(initial_ssthresh, int, 0644);
  67 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  68 module_param(bic_scale, int, 0444);
  69 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  70 module_param(tcp_friendliness, int, 0644);
  71 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  72 module_param(hystart, int, 0644);
  73 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
  74 module_param(hystart_detect, int, 0644);
  75 MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms"
  76                  " 1: packet-train 2: delay 3: both packet-train and delay");
  77 module_param(hystart_low_window, int, 0644);
  78 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
  79 module_param(hystart_ack_delta, int, 0644);
  80 MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
  81
  82 /* BIC TCP Parameters */
  83 struct bictcp {
  84         u32     cnt;            /* increase cwnd by 1 after ACKs */
  85         u32     last_max_cwnd;  /* last maximum snd_cwnd */
  86         u32     loss_cwnd;      /* congestion window at last loss */
  87         u32     last_cwnd;      /* the last snd_cwnd */
  88         u32     last_time;      /* time when updated last_cwnd */
  89         u32     bic_origin_point;/* origin point of bic function */
  90         u32     bic_K;          /* time to origin point
  91                                    from the beginning of the current epoch */
  92         u32     delay_min;      /* min delay (msec << 3) */
  93         u32     epoch_start;    /* beginning of an epoch */
  94         u32     ack_cnt;        /* number of acks */
  95         u32     tcp_cwnd;       /* estimated tcp cwnd */
  96         u16     unused;
  97         u8      sample_cnt;     /* number of samples to decide curr_rtt */
  98         u8      found;          /* the exit point is found? */
  99         u32     round_start;    /* beginning of each round */
 100         u32     end_seq;        /* end_seq of the round */
 101         u32     last_ack;       /* last time when the ACK spacing is close */
 102         u32     curr_rtt;       /* the minimum rtt of current round */
 103 };
 104
 105 static inline void bictcp_reset(struct bictcp *ca)
 106 {
 107         ca->cnt = 0;
 108         ca->last_max_cwnd = 0;
 109         ca->last_cwnd = 0;
 110         ca->last_time = 0;
 111         ca->bic_origin_point = 0;
 112         ca->bic_K = 0;
 113         ca->delay_min = 0;
 114         ca->epoch_start = 0;
 115         ca->ack_cnt = 0;
 116         ca->tcp_cwnd = 0;
 117         ca->found = 0;
 118 }
 119
 120 static inline u32 bictcp_clock(void)
 121 {
 122 #if HZ < 1000
 123         return ktime_to_ms(ktime_get_real());
 124 #else
 125         return jiffies_to_msecs(jiffies);
 126 #endif
 127 }
 128
 129 static inline void bictcp_hystart_reset(struct sock *sk)
 130 {
 131         struct tcp_sock *tp = tcp_sk(sk);
 132         struct bictcp *ca = inet_csk_ca(sk);
 133
 134         ca->round_start = ca->last_ack = bictcp_clock();
 135         ca->end_seq = tp->snd_nxt;
 136         ca->curr_rtt = 0;
 137         ca->sample_cnt = 0;
 138 }
 139
 140 static void bictcp_init(struct sock *sk)
 141 {
 142         struct bictcp *ca = inet_csk_ca(sk);
 143
 144         bictcp_reset(ca);
 145         ca->loss_cwnd = 0;
 146
 147         if (hystart)
 148                 bictcp_hystart_reset(sk);
 149
 150         if (!hystart && initial_ssthresh)
 151                 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 152 }
 153
 154 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 155 {
 156         if (event == CA_EVENT_TX_START) {
 157                 struct bictcp *ca = inet_csk_ca(sk);
 158                 u32 now = tcp_jiffies32;
 159                 s32 delta;
 160
 161                 delta = now - tcp_sk(sk)->lsndtime;
 162
 163                 /* We were application limited (idle) for a while.
 164                  * Shift epoch_start to keep cwnd growth to cubic curve.
 165                  */
 166                 if (ca->epoch_start && delta > 0) {
 167                         ca->epoch_start += delta;
 168                         if (after(ca->epoch_start, now))
 169                                 ca->epoch_start = now;
 170                 }
 171                 return;
 172         }
 173 }
 174
 175 /* calculate the cubic root of x using a table lookup followed by one
 176  * Newton-Raphson iteration.
 177  * Avg err ~= 0.195%
 178  */
 179 static u32 cubic_root(u64 a)
 180 {
 181         u32 x, b, shift;
 182         /*
 183          * cbrt(x) MSB values for x MSB values in [0..63].
 184          * Precomputed then refined by hand - Willy Tarreau
 185          *
 186          * For x in [0..63],
 187          *   v = cbrt(x << 18) - 1
 188          *   cbrt(x) = (v[x] + 10) >> 6
 189          */
 190         static const u8 v[] = {
 191                 /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
 192                 /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
 193                 /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
 194                 /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
 195                 /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
 196                 /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
 197                 /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
 198                 /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
 199         };
 200
 201         b = fls64(a);
 202         if (b < 7) {
 203                 /* a in [0..63] */
 204                 return ((u32)v[(u32)a] + 35) >> 6;
 205         }
 206
 207         b = ((b * 84) >> 8) - 1;
 208         shift = (a >> (b * 3));
 209
 210         x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
 211
 212         /*
 213          * Newton-Raphson iteration
 214          *                         2
 215          * x    = ( 2 * x  +  a / x  ) / 3
 216          *  k+1          k         k
 217          */
 218         x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
 219         x = ((x * 341) >> 10);
 220         return x;
 221 }
 222
 223 /*
 224  * Compute congestion window to use.
 225  */
 226 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 227 {
 228         u32 delta, bic_target, max_cnt;
 229         u64 offs, t;
 230
 231         ca->ack_cnt += acked;   /* count the number of ACKed packets */
 232
 233         if (ca->last_cwnd == cwnd &&
 234             (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
 235                 return;
 236
 237         /* The CUBIC function can update ca->cnt at most once per jiffy.
 238          * On all cwnd reduction events, ca->epoch_start is set to 0,
 239          * which will force a recalculation of ca->cnt.
 240          */
 241         if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
 242                 goto tcp_friendliness;
 243
 244         ca->last_cwnd = cwnd;
 245         ca->last_time = tcp_jiffies32;
 246
 247         if (ca->epoch_start == 0) {
 248                 ca->epoch_start = tcp_jiffies32;        /* record beginning */
 249                 ca->ack_cnt = acked;                    /* start counting */
 250                 ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 251
 252                 if (ca->last_max_cwnd <= cwnd) {
 253                         ca->bic_K = 0;
 254                         ca->bic_origin_point = cwnd;
 255                 } else {
 256                         /* Compute new K based on
 257                          * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 258                          */
 259                         ca->bic_K = cubic_root(cube_factor
 260                                                * (ca->last_max_cwnd - cwnd));
 261                         ca->bic_origin_point = ca->last_max_cwnd;
 262                 }
 263         }
 264
 265         /* cubic function - calc*/
 266         /* calculate c * time^3 / rtt,
 267          *  while considering overflow in calculation of time^3
 268          * (so time^3 is done by using 64 bit)
 269          * and without the support of division of 64bit numbers
 270          * (so all divisions are done by using 32 bit)
 271          *  also NOTE the unit of those veriables
 272          *        time  = (t - K) / 2^bictcp_HZ
 273          *        c = bic_scale >> 10
 274          * rtt  = (srtt >> 3) / HZ
 275          * !!! The following code does not have overflow problems,
 276          * if the cwnd < 1 million packets !!!
 277          */
 278
 279         t = (s32)(tcp_jiffies32 - ca->epoch_start);
 280         t += msecs_to_jiffies(ca->delay_min >> 3);
 281         /* change the unit from HZ to bictcp_HZ */
 282         t <<= BICTCP_HZ;
 283         do_div(t, HZ);
 284
 285         if (t < ca->bic_K)              /* t - K */
 286                 offs = ca->bic_K - t;
 287         else
 288                 offs = t - ca->bic_K;
 289
 290         /* c/rtt * (t-K)^3 */
 291         delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 292         if (t < ca->bic_K)                            /* below origin*/
 293                 bic_target = ca->bic_origin_point - delta;
 294         else                                          /* above origin*/
 295                 bic_target = ca->bic_origin_point + delta;
 296
 297         /* cubic function - calc bictcp_cnt*/
 298         if (bic_target > cwnd) {
 299                 ca->cnt = cwnd / (bic_target - cwnd);
 300         } else {
 301                 ca->cnt = 100 * cwnd;              /* very small increment*/
 302         }
 303
 304         /*
 305          * The initial growth of cubic function may be too conservative
 306          * when the available bandwidth is still unknown.
 307          */
 308         if (ca->last_max_cwnd == 0 && ca->cnt > 20)
 309                 ca->cnt = 20;   /* increase cwnd 5% per RTT */
 310
 311 tcp_friendliness:
 312         /* TCP Friendly */
 313         if (tcp_friendliness) {
 314                 u32 scale = beta_scale;
 315
 316                 delta = (cwnd * scale) >> 3;
 317                 while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 318                         ca->ack_cnt -= delta;
 319                         ca->tcp_cwnd++;
 320                 }
 321
 322                 if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
 323                         delta = ca->tcp_cwnd - cwnd;
 324                         max_cnt = cwnd / delta;
 325                         if (ca->cnt > max_cnt)
 326                                 ca->cnt = max_cnt;
 327                 }
 328         }
 329
 330         /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
 331          * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
 332          */
 333         ca->cnt = max(ca->cnt, 2U);
 334 }
 335
 336 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 337 {
 338         struct tcp_sock *tp = tcp_sk(sk);
 339         struct bictcp *ca = inet_csk_ca(sk);
 340
 341         if (!tcp_is_cwnd_limited(sk))
 342                 return;
 343
 344         if (tcp_in_slow_start(tp)) {
 345                 if (hystart && after(ack, ca->end_seq))
 346                         bictcp_hystart_reset(sk);
 347                 acked = tcp_slow_start(tp, acked);
 348                 if (!acked)
 349                         return;
 350         }
 351         bictcp_update(ca, tp->snd_cwnd, acked);
 352         tcp_cong_avoid_ai(tp, ca->cnt, acked);
 353 }
 354
 355 static u32 bictcp_recalc_ssthresh(struct sock *sk)
 356 {
 357         const struct tcp_sock *tp = tcp_sk(sk);
 358         struct bictcp *ca = inet_csk_ca(sk);
 359
 360         ca->epoch_start = 0;    /* end of epoch */
 361
 362         /* Wmax and fast convergence */
 363         if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 364                 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 365                         / (2 * BICTCP_BETA_SCALE);
 366         else
 367                 ca->last_max_cwnd = tp->snd_cwnd;
 368
 369         ca->loss_cwnd = tp->snd_cwnd;
 370
 371         return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 372 }
 373
 374 static u32 bictcp_undo_cwnd(struct sock *sk)
 375 {
 376         struct bictcp *ca = inet_csk_ca(sk);
 377
 378         return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
 379 }
 380
 381 static void bictcp_state(struct sock *sk, u8 new_state)
 382 {
 383         if (new_state == TCP_CA_Loss) {
 384                 bictcp_reset(inet_csk_ca(sk));
 385                 bictcp_hystart_reset(sk);
 386         }
 387 }
 388
 389 static void hystart_update(struct sock *sk, u32 delay)
 390 {
 391         struct tcp_sock *tp = tcp_sk(sk);
 392         struct bictcp *ca = inet_csk_ca(sk);
 393
 394         if (ca->found & hystart_detect)
 395                 return;
 396
 397         if (hystart_detect & HYSTART_ACK_TRAIN) {
 398                 u32 now = bictcp_clock();
 399
 400                 /* first detection parameter - ack-train detection */
 401                 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
 402                         ca->last_ack = now;
 403                         if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
 404                                 ca->found |= HYSTART_ACK_TRAIN;
 405                                 NET_INC_STATS(sock_net(sk),
 406                                               LINUX_MIB_TCPHYSTARTTRAINDETECT);
 407                                 NET_ADD_STATS(sock_net(sk),
 408                                               LINUX_MIB_TCPHYSTARTTRAINCWND,
 409                                               tp->snd_cwnd);
 410                                 tp->snd_ssthresh = tp->snd_cwnd;
 411                         }
 412                 }
 413         }
 414
 415         if (hystart_detect & HYSTART_DELAY) {
 416                 /* obtain the minimum delay of more than sampling packets */
 417                 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
 418                         if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
 419                                 ca->curr_rtt = delay;
 420
 421                         ca->sample_cnt++;
 422                 } else {
 423                         if (ca->curr_rtt > ca->delay_min +
 424                             HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
 425                                 ca->found |= HYSTART_DELAY;
 426                                 NET_INC_STATS(sock_net(sk),
 427                                               LINUX_MIB_TCPHYSTARTDELAYDETECT);
 428                                 NET_ADD_STATS(sock_net(sk),
 429                                               LINUX_MIB_TCPHYSTARTDELAYCWND,
 430                                               tp->snd_cwnd);
 431                                 tp->snd_ssthresh = tp->snd_cwnd;
 432                         }
 433                 }
 434         }
 435 }
 436
 437 /* Track delayed acknowledgment ratio using sliding window
 438  * ratio = (15*ratio + sample) / 16
 439  */
 440 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
 441 {
 442         const struct tcp_sock *tp = tcp_sk(sk);
 443         struct bictcp *ca = inet_csk_ca(sk);
 444         u32 delay;
 445
 446         /* Some calls are for duplicates without timetamps */
 447         if (sample->rtt_us < 0)
 448                 return;
 449
 450         /* Discard delay samples right after fast recovery */
 451         if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
 452                 return;
 453
 454         delay = (sample->rtt_us << 3) / USEC_PER_MSEC;
 455         if (delay == 0)
 456                 delay = 1;
 457
 458         /* first time call or link delay decreases */
 459         if (ca->delay_min == 0 || ca->delay_min > delay)
 460                 ca->delay_min = delay;
 461
 462         /* hystart triggers when cwnd is larger than some threshold */
 463         if (hystart && tcp_in_slow_start(tp) &&
 464             tp->snd_cwnd >= hystart_low_window)
 465                 hystart_update(sk, delay);
 466 }
 467
 468 static struct tcp_congestion_ops cubictcp __read_mostly = {
 469         .init           = bictcp_init,
 470         .ssthresh       = bictcp_recalc_ssthresh,
 471         .cong_avoid     = bictcp_cong_avoid,
 472         .set_state      = bictcp_state,
 473         .undo_cwnd      = bictcp_undo_cwnd,
 474         .cwnd_event     = bictcp_cwnd_event,
 475         .pkts_acked     = bictcp_acked,
 476         .owner          = THIS_MODULE,
 477         .name           = "cubic",
 478 };
 479
 480 static int __init cubictcp_register(void)
 481 {
 482         BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 483
 484         /* Precompute a bunch of the scaling factors that are used per-packet
 485          * based on SRTT of 100ms
 486          */
 487
 488         beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
 489                 / (BICTCP_BETA_SCALE - beta);
 490
 491         cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 492
 493         /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 494          *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 495          * the unit of K is bictcp_HZ=2^10, not HZ
 496          *
 497          *  c = bic_scale >> 10
 498          *  rtt = 100ms
 499          *
 500          * the following code has been designed and tested for
 501          * cwnd < 1 million packets
 502          * RTT < 100 seconds
 503          * HZ < 1,000,00  (corresponding to 10 nano-second)
 504          */
 505
 506         /* 1/c * 2^2*bictcp_HZ * srtt */
 507         cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 508
 509         /* divide by bic_scale and by constant Srtt (100ms) */
 510         do_div(cube_factor, bic_scale * 10);
 511
 512         return tcp_register_congestion_control(&cubictcp);
 513 }
 514
 515 static void __exit cubictcp_unregister(void)
 516 {
 517         tcp_unregister_congestion_control(&cubictcp);
 518 }
 519
 520 module_init(cubictcp_register);
 521 module_exit(cubictcp_unregister);
 522
 523 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 524 MODULE_LICENSE("GPL");
 525 MODULE_DESCRIPTION("CUBIC TCP");
 526 MODULE_VERSION("2.3");