net/ipv4/tcp_cubic.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
   4  * Home page:
   5  *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
   6  * This is from the implementation of CUBIC TCP in
   7  * Sangtae Ha, Injong Rhee and Lisong Xu,
   8  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
   9  *  in ACM SIGOPS Operating System Review, July 2008.
  10  * Available from:
  11  *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
  12  *
  13  * CUBIC integrates a new slow start algorithm, called HyStart.
  14  * The details of HyStart are presented in
  15  *  Sangtae Ha and Injong Rhee,
  16  *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
  17  * Available from:
  18  *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
  19  *
  20  * All testing results are available from:
  21  * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
  22  *
  23  * Unless CUBIC is enabled and congestion window is large
  24  * this behaves the same as the original Reno.
  25  */
  26
  27 #include <linux/mm.h>
  28 #include <linux/module.h>
  29 #include <linux/math64.h>
  30 #include <net/tcp.h>
  31
  32 #define BICTCP_BETA_SCALE    1024       /* Scale factor beta calculation
  33                                          * max_cwnd = snd_cwnd * beta
  34                                          */
  35 #define BICTCP_HZ               10      /* BIC HZ 2^10 = 1024 */
  36
  37 /* Two methods of hybrid slow start */
  38 #define HYSTART_ACK_TRAIN       0x1
  39 #define HYSTART_DELAY           0x2
  40
  41 /* Number of delay samples for detecting the increase of delay */
  42 #define HYSTART_MIN_SAMPLES     8
  43 #define HYSTART_DELAY_MIN       (4U<<3)
  44 #define HYSTART_DELAY_MAX       (16U<<3)
  45 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
  46
  47 static int fast_convergence __read_mostly = 1;
  48 static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
  49 static int initial_ssthresh __read_mostly;
  50 static int bic_scale __read_mostly = 41;
  51 static int tcp_friendliness __read_mostly = 1;
  52
  53 static int hystart __read_mostly = 1;
  54 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
  55 static int hystart_low_window __read_mostly = 16;
  56 static int hystart_ack_delta __read_mostly = 2;
  57
  58 static u32 cube_rtt_scale __read_mostly;
  59 static u32 beta_scale __read_mostly;
  60 static u64 cube_factor __read_mostly;
  61
  62 /* Note parameters that are used for precomputing scale factors are read-only */
  63 module_param(fast_convergence, int, 0644);
  64 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  65 module_param(beta, int, 0644);
  66 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  67 module_param(initial_ssthresh, int, 0644);
  68 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  69 module_param(bic_scale, int, 0444);
  70 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  71 module_param(tcp_friendliness, int, 0644);
  72 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  73 module_param(hystart, int, 0644);
  74 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
  75 module_param(hystart_detect, int, 0644);
  76 MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms"
  77                  " 1: packet-train 2: delay 3: both packet-train and delay");
  78 module_param(hystart_low_window, int, 0644);
  79 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
  80 module_param(hystart_ack_delta, int, 0644);
  81 MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
  82
  83 /* BIC TCP Parameters */
  84 struct bictcp {
  85         u32     cnt;            /* increase cwnd by 1 after ACKs */
  86         u32     last_max_cwnd;  /* last maximum snd_cwnd */
  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
 146         if (hystart)
 147                 bictcp_hystart_reset(sk);
 148
 149         if (!hystart && initial_ssthresh)
 150                 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 151 }
 152
 153 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
 154 {
 155         if (event == CA_EVENT_TX_START) {
 156                 struct bictcp *ca = inet_csk_ca(sk);
 157                 u32 now = tcp_jiffies32;
 158                 s32 delta;
 159
 160                 delta = now - tcp_sk(sk)->lsndtime;
 161
 162                 /* We were application limited (idle) for a while.
 163                  * Shift epoch_start to keep cwnd growth to cubic curve.
 164                  */
 165                 if (ca->epoch_start && delta > 0) {
 166                         ca->epoch_start += delta;
 167                         if (after(ca->epoch_start, now))
 168                                 ca->epoch_start = now;
 169                 }
 170                 return;
 171         }
 172 }
 173
 174 /* calculate the cubic root of x using a table lookup followed by one
 175  * Newton-Raphson iteration.
 176  * Avg err ~= 0.195%
 177  */
 178 static u32 cubic_root(u64 a)
 179 {
 180         u32 x, b, shift;
 181         /*
 182          * cbrt(x) MSB values for x MSB values in [0..63].
 183          * Precomputed then refined by hand - Willy Tarreau
 184          *
 185          * For x in [0..63],
 186          *   v = cbrt(x << 18) - 1
 187          *   cbrt(x) = (v[x] + 10) >> 6
 188          */
 189         static const u8 v[] = {
 190                 /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
 191                 /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
 192                 /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
 193                 /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
 194                 /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
 195                 /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
 196                 /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
 197                 /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
 198         };
 199
 200         b = fls64(a);
 201         if (b < 7) {
 202                 /* a in [0..63] */
 203                 return ((u32)v[(u32)a] + 35) >> 6;
 204         }
 205
 206         b = ((b * 84) >> 8) - 1;
 207         shift = (a >> (b * 3));
 208
 209         x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
 210
 211         /*
 212          * Newton-Raphson iteration
 213          *                         2
 214          * x    = ( 2 * x  +  a / x  ) / 3
 215          *  k+1          k         k
 216          */
 217         x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
 218         x = ((x * 341) >> 10);
 219         return x;
 220 }
 221
 222 /*
 223  * Compute congestion window to use.
 224  */
 225 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 226 {
 227         u32 delta, bic_target, max_cnt;
 228         u64 offs, t;
 229
 230         ca->ack_cnt += acked;   /* count the number of ACKed packets */
 231
 232         if (ca->last_cwnd == cwnd &&
 233             (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
 234                 return;
 235
 236         /* The CUBIC function can update ca->cnt at most once per jiffy.
 237          * On all cwnd reduction events, ca->epoch_start is set to 0,
 238          * which will force a recalculation of ca->cnt.
 239          */
 240         if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
 241                 goto tcp_friendliness;
 242
 243         ca->last_cwnd = cwnd;
 244         ca->last_time = tcp_jiffies32;
 245
 246         if (ca->epoch_start == 0) {
 247                 ca->epoch_start = tcp_jiffies32;        /* record beginning */
 248                 ca->ack_cnt = acked;                    /* start counting */
 249                 ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 250
 251                 if (ca->last_max_cwnd <= cwnd) {
 252                         ca->bic_K = 0;
 253                         ca->bic_origin_point = cwnd;
 254                 } else {
 255                         /* Compute new K based on
 256                          * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 257                          */
 258                         ca->bic_K = cubic_root(cube_factor
 259                                                * (ca->last_max_cwnd - cwnd));
 260                         ca->bic_origin_point = ca->last_max_cwnd;
 261                 }
 262         }
 263
 264         /* cubic function - calc*/
 265         /* calculate c * time^3 / rtt,
 266          *  while considering overflow in calculation of time^3
 267          * (so time^3 is done by using 64 bit)
 268          * and without the support of division of 64bit numbers
 269          * (so all divisions are done by using 32 bit)
 270          *  also NOTE the unit of those veriables
 271          *        time  = (t - K) / 2^bictcp_HZ
 272          *        c = bic_scale >> 10
 273          * rtt  = (srtt >> 3) / HZ
 274          * !!! The following code does not have overflow problems,
 275          * if the cwnd < 1 million packets !!!
 276          */
 277
 278         t = (s32)(tcp_jiffies32 - ca->epoch_start);
 279         t += msecs_to_jiffies(ca->delay_min >> 3);
 280         /* change the unit from HZ to bictcp_HZ */
 281         t <<= BICTCP_HZ;
 282         do_div(t, HZ);
 283
 284         if (t < ca->bic_K)              /* t - K */
 285                 offs = ca->bic_K - t;
 286         else
 287                 offs = t - ca->bic_K;
 288
 289         /* c/rtt * (t-K)^3 */
 290         delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 291         if (t < ca->bic_K)                            /* below origin*/
 292                 bic_target = ca->bic_origin_point - delta;
 293         else                                          /* above origin*/
 294                 bic_target = ca->bic_origin_point + delta;
 295
 296         /* cubic function - calc bictcp_cnt*/
 297         if (bic_target > cwnd) {
 298                 ca->cnt = cwnd / (bic_target - cwnd);
 299         } else {
 300                 ca->cnt = 100 * cwnd;              /* very small increment*/
 301         }
 302
 303         /*
 304          * The initial growth of cubic function may be too conservative
 305          * when the available bandwidth is still unknown.
 306          */
 307         if (ca->last_max_cwnd == 0 && ca->cnt > 20)
 308                 ca->cnt = 20;   /* increase cwnd 5% per RTT */
 309
 310 tcp_friendliness:
 311         /* TCP Friendly */
 312         if (tcp_friendliness) {
 313                 u32 scale = beta_scale;
 314
 315                 delta = (cwnd * scale) >> 3;
 316                 while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 317                         ca->ack_cnt -= delta;
 318                         ca->tcp_cwnd++;
 319                 }
 320
 321                 if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
 322                         delta = ca->tcp_cwnd - cwnd;
 323                         max_cnt = cwnd / delta;
 324                         if (ca->cnt > max_cnt)
 325                                 ca->cnt = max_cnt;
 326                 }
 327         }
 328
 329         /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
 330          * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
 331          */
 332         ca->cnt = max(ca->cnt, 2U);
 333 }
 334
 335 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 336 {
 337         struct tcp_sock *tp = tcp_sk(sk);
 338         struct bictcp *ca = inet_csk_ca(sk);
 339
 340         if (!tcp_is_cwnd_limited(sk))
 341                 return;
 342
 343         if (tcp_in_slow_start(tp)) {
 344                 if (hystart && after(ack, ca->end_seq))
 345                         bictcp_hystart_reset(sk);
 346                 acked = tcp_slow_start(tp, acked);
 347                 if (!acked)
 348                         return;
 349         }
 350         bictcp_update(ca, tp->snd_cwnd, acked);
 351         tcp_cong_avoid_ai(tp, ca->cnt, acked);
 352 }
 353
 354 static u32 bictcp_recalc_ssthresh(struct sock *sk)
 355 {
 356         const struct tcp_sock *tp = tcp_sk(sk);
 357         struct bictcp *ca = inet_csk_ca(sk);
 358
 359         ca->epoch_start = 0;    /* end of epoch */
 360
 361         /* Wmax and fast convergence */
 362         if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 363                 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 364                         / (2 * BICTCP_BETA_SCALE);
 365         else
 366                 ca->last_max_cwnd = tp->snd_cwnd;
 367
 368         return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 369 }
 370
 371 static void bictcp_state(struct sock *sk, u8 new_state)
 372 {
 373         if (new_state == TCP_CA_Loss) {
 374                 bictcp_reset(inet_csk_ca(sk));
 375                 bictcp_hystart_reset(sk);
 376         }
 377 }
 378
 379 static void hystart_update(struct sock *sk, u32 delay)
 380 {
 381         struct tcp_sock *tp = tcp_sk(sk);
 382         struct bictcp *ca = inet_csk_ca(sk);
 383
 384         if (ca->found & hystart_detect)
 385                 return;
 386
 387         if (hystart_detect & HYSTART_ACK_TRAIN) {
 388                 u32 now = bictcp_clock();
 389
 390                 /* first detection parameter - ack-train detection */
 391                 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
 392                         ca->last_ack = now;
 393                         if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
 394                                 ca->found |= HYSTART_ACK_TRAIN;
 395                                 NET_INC_STATS(sock_net(sk),
 396                                               LINUX_MIB_TCPHYSTARTTRAINDETECT);
 397                                 NET_ADD_STATS(sock_net(sk),
 398                                               LINUX_MIB_TCPHYSTARTTRAINCWND,
 399                                               tp->snd_cwnd);
 400                                 tp->snd_ssthresh = tp->snd_cwnd;
 401                         }
 402                 }
 403         }
 404
 405         if (hystart_detect & HYSTART_DELAY) {
 406                 /* obtain the minimum delay of more than sampling packets */
 407                 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
 408                         if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
 409                                 ca->curr_rtt = delay;
 410
 411                         ca->sample_cnt++;
 412                 } else {
 413                         if (ca->curr_rtt > ca->delay_min +
 414                             HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
 415                                 ca->found |= HYSTART_DELAY;
 416                                 NET_INC_STATS(sock_net(sk),
 417                                               LINUX_MIB_TCPHYSTARTDELAYDETECT);
 418                                 NET_ADD_STATS(sock_net(sk),
 419                                               LINUX_MIB_TCPHYSTARTDELAYCWND,
 420                                               tp->snd_cwnd);
 421                                 tp->snd_ssthresh = tp->snd_cwnd;
 422                         }
 423                 }
 424         }
 425 }
 426
 427 /* Track delayed acknowledgment ratio using sliding window
 428  * ratio = (15*ratio + sample) / 16
 429  */
 430 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
 431 {
 432         const struct tcp_sock *tp = tcp_sk(sk);
 433         struct bictcp *ca = inet_csk_ca(sk);
 434         u32 delay;
 435
 436         /* Some calls are for duplicates without timetamps */
 437         if (sample->rtt_us < 0)
 438                 return;
 439
 440         /* Discard delay samples right after fast recovery */
 441         if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
 442                 return;
 443
 444         delay = (sample->rtt_us << 3) / USEC_PER_MSEC;
 445         if (delay == 0)
 446                 delay = 1;
 447
 448         /* first time call or link delay decreases */
 449         if (ca->delay_min == 0 || ca->delay_min > delay)
 450                 ca->delay_min = delay;
 451
 452         /* hystart triggers when cwnd is larger than some threshold */
 453         if (hystart && tcp_in_slow_start(tp) &&
 454             tp->snd_cwnd >= hystart_low_window)
 455                 hystart_update(sk, delay);
 456 }
 457
 458 static struct tcp_congestion_ops cubictcp __read_mostly = {
 459         .init           = bictcp_init,
 460         .ssthresh       = bictcp_recalc_ssthresh,
 461         .cong_avoid     = bictcp_cong_avoid,
 462         .set_state      = bictcp_state,
 463         .undo_cwnd      = tcp_reno_undo_cwnd,
 464         .cwnd_event     = bictcp_cwnd_event,
 465         .pkts_acked     = bictcp_acked,
 466         .owner          = THIS_MODULE,
 467         .name           = "cubic",
 468 };
 469
 470 static int __init cubictcp_register(void)
 471 {
 472         BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 473
 474         /* Precompute a bunch of the scaling factors that are used per-packet
 475          * based on SRTT of 100ms
 476          */
 477
 478         beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
 479                 / (BICTCP_BETA_SCALE - beta);
 480
 481         cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 482
 483         /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 484          *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 485          * the unit of K is bictcp_HZ=2^10, not HZ
 486          *
 487          *  c = bic_scale >> 10
 488          *  rtt = 100ms
 489          *
 490          * the following code has been designed and tested for
 491          * cwnd < 1 million packets
 492          * RTT < 100 seconds
 493          * HZ < 1,000,00  (corresponding to 10 nano-second)
 494          */
 495
 496         /* 1/c * 2^2*bictcp_HZ * srtt */
 497         cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 498
 499         /* divide by bic_scale and by constant Srtt (100ms) */
 500         do_div(cube_factor, bic_scale * 10);
 501
 502         return tcp_register_congestion_control(&cubictcp);
 503 }
 504
 505 static void __exit cubictcp_unregister(void)
 506 {
 507         tcp_unregister_congestion_control(&cubictcp);
 508 }
 509
 510 module_init(cubictcp_register);
 511 module_exit(cubictcp_unregister);
 512
 513 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 514 MODULE_LICENSE("GPL");
 515 MODULE_DESCRIPTION("CUBIC TCP");
 516 MODULE_VERSION("2.3");