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, "hyrbrid 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 #define ACK_RATIO_SHIFT 4
  97 #define ACK_RATIO_LIMIT (32u << ACK_RATIO_SHIFT)
  98         u16     delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
  99         u8      sample_cnt;     /* number of samples to decide curr_rtt */
 100         u8      found;          /* the exit point is found? */
 101         u32     round_start;    /* beginning of each round */
 102         u32     end_seq;        /* end_seq of the round */
 103         u32     last_ack;       /* last time when the ACK spacing is close */
 104         u32     curr_rtt;       /* the minimum rtt of current round */
 105 };
 106
 107 static inline void bictcp_reset(struct bictcp *ca)
 108 {
 109         ca->cnt = 0;
 110         ca->last_max_cwnd = 0;
 111         ca->last_cwnd = 0;
 112         ca->last_time = 0;
 113         ca->bic_origin_point = 0;
 114         ca->bic_K = 0;
 115         ca->delay_min = 0;
 116         ca->epoch_start = 0;
 117         ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
 118         ca->ack_cnt = 0;
 119         ca->tcp_cwnd = 0;
 120         ca->found = 0;
 121 }
 122
 123 static inline u32 bictcp_clock(void)
 124 {
 125 #if HZ < 1000
 126         return ktime_to_ms(ktime_get_real());
 127 #else
 128         return jiffies_to_msecs(jiffies);
 129 #endif
 130 }
 131
 132 static inline void bictcp_hystart_reset(struct sock *sk)
 133 {
 134         struct tcp_sock *tp = tcp_sk(sk);
 135         struct bictcp *ca = inet_csk_ca(sk);
 136
 137         ca->round_start = ca->last_ack = bictcp_clock();
 138         ca->end_seq = tp->snd_nxt;
 139         ca->curr_rtt = 0;
 140         ca->sample_cnt = 0;
 141 }
 142
 143 static void bictcp_init(struct sock *sk)
 144 {
 145         struct bictcp *ca = inet_csk_ca(sk);
 146
 147         bictcp_reset(ca);
 148         ca->loss_cwnd = 0;
 149
 150         if (hystart)
 151                 bictcp_hystart_reset(sk);
 152
 153         if (!hystart && initial_ssthresh)
 154                 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 155 }
 156
 157 /* calculate the cubic root of x using a table lookup followed by one
 158  * Newton-Raphson iteration.
 159  * Avg err ~= 0.195%
 160  */
 161 static u32 cubic_root(u64 a)
 162 {
 163         u32 x, b, shift;
 164         /*
 165          * cbrt(x) MSB values for x MSB values in [0..63].
 166          * Precomputed then refined by hand - Willy Tarreau
 167          *
 168          * For x in [0..63],
 169          *   v = cbrt(x << 18) - 1
 170          *   cbrt(x) = (v[x] + 10) >> 6
 171          */
 172         static const u8 v[] = {
 173                 /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
 174                 /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
 175                 /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
 176                 /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
 177                 /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
 178                 /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
 179                 /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
 180                 /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
 181         };
 182
 183         b = fls64(a);
 184         if (b < 7) {
 185                 /* a in [0..63] */
 186                 return ((u32)v[(u32)a] + 35) >> 6;
 187         }
 188
 189         b = ((b * 84) >> 8) - 1;
 190         shift = (a >> (b * 3));
 191
 192         x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
 193
 194         /*
 195          * Newton-Raphson iteration
 196          *                         2
 197          * x    = ( 2 * x  +  a / x  ) / 3
 198          *  k+1          k         k
 199          */
 200         x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
 201         x = ((x * 341) >> 10);
 202         return x;
 203 }
 204
 205 /*
 206  * Compute congestion window to use.
 207  */
 208 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 209 {
 210         u32 delta, bic_target, max_cnt;
 211         u64 offs, t;
 212
 213         ca->ack_cnt++;  /* count the number of ACKs */
 214
 215         if (ca->last_cwnd == cwnd &&
 216             (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
 217                 return;
 218
 219         ca->last_cwnd = cwnd;
 220         ca->last_time = tcp_time_stamp;
 221
 222         if (ca->epoch_start == 0) {
 223                 ca->epoch_start = tcp_time_stamp;       /* record beginning */
 224                 ca->ack_cnt = 1;                        /* start counting */
 225                 ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 226
 227                 if (ca->last_max_cwnd <= cwnd) {
 228                         ca->bic_K = 0;
 229                         ca->bic_origin_point = cwnd;
 230                 } else {
 231                         /* Compute new K based on
 232                          * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 233                          */
 234                         ca->bic_K = cubic_root(cube_factor
 235                                                * (ca->last_max_cwnd - cwnd));
 236                         ca->bic_origin_point = ca->last_max_cwnd;
 237                 }
 238         }
 239
 240         /* cubic function - calc*/
 241         /* calculate c * time^3 / rtt,
 242          *  while considering overflow in calculation of time^3
 243          * (so time^3 is done by using 64 bit)
 244          * and without the support of division of 64bit numbers
 245          * (so all divisions are done by using 32 bit)
 246          *  also NOTE the unit of those veriables
 247          *        time  = (t - K) / 2^bictcp_HZ
 248          *        c = bic_scale >> 10
 249          * rtt  = (srtt >> 3) / HZ
 250          * !!! The following code does not have overflow problems,
 251          * if the cwnd < 1 million packets !!!
 252          */
 253
 254         t = (s32)(tcp_time_stamp - ca->epoch_start);
 255         t += msecs_to_jiffies(ca->delay_min >> 3);
 256         /* change the unit from HZ to bictcp_HZ */
 257         t <<= BICTCP_HZ;
 258         do_div(t, HZ);
 259
 260         if (t < ca->bic_K)              /* t - K */
 261                 offs = ca->bic_K - t;
 262         else
 263                 offs = t - ca->bic_K;
 264
 265         /* c/rtt * (t-K)^3 */
 266         delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 267         if (t < ca->bic_K)                            /* below origin*/
 268                 bic_target = ca->bic_origin_point - delta;
 269         else                                          /* above origin*/
 270                 bic_target = ca->bic_origin_point + delta;
 271
 272         /* cubic function - calc bictcp_cnt*/
 273         if (bic_target > cwnd) {
 274                 ca->cnt = cwnd / (bic_target - cwnd);
 275         } else {
 276                 ca->cnt = 100 * cwnd;              /* very small increment*/
 277         }
 278
 279         /*
 280          * The initial growth of cubic function may be too conservative
 281          * when the available bandwidth is still unknown.
 282          */
 283         if (ca->last_max_cwnd == 0 && ca->cnt > 20)
 284                 ca->cnt = 20;   /* increase cwnd 5% per RTT */
 285
 286         /* TCP Friendly */
 287         if (tcp_friendliness) {
 288                 u32 scale = beta_scale;
 289
 290                 delta = (cwnd * scale) >> 3;
 291                 while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 292                         ca->ack_cnt -= delta;
 293                         ca->tcp_cwnd++;
 294                 }
 295
 296                 if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
 297                         delta = ca->tcp_cwnd - cwnd;
 298                         max_cnt = cwnd / delta;
 299                         if (ca->cnt > max_cnt)
 300                                 ca->cnt = max_cnt;
 301                 }
 302         }
 303
 304         ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
 305         if (ca->cnt == 0)                       /* cannot be zero */
 306                 ca->cnt = 1;
 307 }
 308
 309 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 310 {
 311         struct tcp_sock *tp = tcp_sk(sk);
 312         struct bictcp *ca = inet_csk_ca(sk);
 313
 314         if (!tcp_is_cwnd_limited(sk))
 315                 return;
 316
 317         if (tp->snd_cwnd <= tp->snd_ssthresh) {
 318                 if (hystart && after(ack, ca->end_seq))
 319                         bictcp_hystart_reset(sk);
 320                 tcp_slow_start(tp, acked);
 321         } else {
 322                 bictcp_update(ca, tp->snd_cwnd);
 323                 tcp_cong_avoid_ai(tp, ca->cnt);
 324         }
 325 }
 326
 327 static u32 bictcp_recalc_ssthresh(struct sock *sk)
 328 {
 329         const struct tcp_sock *tp = tcp_sk(sk);
 330         struct bictcp *ca = inet_csk_ca(sk);
 331
 332         ca->epoch_start = 0;    /* end of epoch */
 333
 334         /* Wmax and fast convergence */
 335         if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 336                 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 337                         / (2 * BICTCP_BETA_SCALE);
 338         else
 339                 ca->last_max_cwnd = tp->snd_cwnd;
 340
 341         ca->loss_cwnd = tp->snd_cwnd;
 342
 343         return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 344 }
 345
 346 static u32 bictcp_undo_cwnd(struct sock *sk)
 347 {
 348         struct bictcp *ca = inet_csk_ca(sk);
 349
 350         return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
 351 }
 352
 353 static void bictcp_state(struct sock *sk, u8 new_state)
 354 {
 355         if (new_state == TCP_CA_Loss) {
 356                 bictcp_reset(inet_csk_ca(sk));
 357                 bictcp_hystart_reset(sk);
 358         }
 359 }
 360
 361 static void hystart_update(struct sock *sk, u32 delay)
 362 {
 363         struct tcp_sock *tp = tcp_sk(sk);
 364         struct bictcp *ca = inet_csk_ca(sk);
 365
 366         if (ca->found & hystart_detect)
 367                 return;
 368
 369         if (hystart_detect & HYSTART_ACK_TRAIN) {
 370                 u32 now = bictcp_clock();
 371
 372                 /* first detection parameter - ack-train detection */
 373                 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
 374                         ca->last_ack = now;
 375                         if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
 376                                 ca->found |= HYSTART_ACK_TRAIN;
 377                                 NET_INC_STATS_BH(sock_net(sk),
 378                                                  LINUX_MIB_TCPHYSTARTTRAINDETECT);
 379                                 NET_ADD_STATS_BH(sock_net(sk),
 380                                                  LINUX_MIB_TCPHYSTARTTRAINCWND,
 381                                                  tp->snd_cwnd);
 382                                 tp->snd_ssthresh = tp->snd_cwnd;
 383                         }
 384                 }
 385         }
 386
 387         if (hystart_detect & HYSTART_DELAY) {
 388                 /* obtain the minimum delay of more than sampling packets */
 389                 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
 390                         if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
 391                                 ca->curr_rtt = delay;
 392
 393                         ca->sample_cnt++;
 394                 } else {
 395                         if (ca->curr_rtt > ca->delay_min +
 396                             HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
 397                                 ca->found |= HYSTART_DELAY;
 398                                 NET_INC_STATS_BH(sock_net(sk),
 399                                                  LINUX_MIB_TCPHYSTARTDELAYDETECT);
 400                                 NET_ADD_STATS_BH(sock_net(sk),
 401                                                  LINUX_MIB_TCPHYSTARTDELAYCWND,
 402                                                  tp->snd_cwnd);
 403                                 tp->snd_ssthresh = tp->snd_cwnd;
 404                         }
 405                 }
 406         }
 407 }
 408
 409 /* Track delayed acknowledgment ratio using sliding window
 410  * ratio = (15*ratio + sample) / 16
 411  */
 412 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 413 {
 414         const struct inet_connection_sock *icsk = inet_csk(sk);
 415         const struct tcp_sock *tp = tcp_sk(sk);
 416         struct bictcp *ca = inet_csk_ca(sk);
 417         u32 delay;
 418
 419         if (icsk->icsk_ca_state == TCP_CA_Open) {
 420                 u32 ratio = ca->delayed_ack;
 421
 422                 ratio -= ca->delayed_ack >> ACK_RATIO_SHIFT;
 423                 ratio += cnt;
 424
 425                 ca->delayed_ack = clamp(ratio, 1U, ACK_RATIO_LIMIT);
 426         }
 427
 428         /* Some calls are for duplicates without timetamps */
 429         if (rtt_us < 0)
 430                 return;
 431
 432         /* Discard delay samples right after fast recovery */
 433         if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 434                 return;
 435
 436         delay = (rtt_us << 3) / USEC_PER_MSEC;
 437         if (delay == 0)
 438                 delay = 1;
 439
 440         /* first time call or link delay decreases */
 441         if (ca->delay_min == 0 || ca->delay_min > delay)
 442                 ca->delay_min = delay;
 443
 444         /* hystart triggers when cwnd is larger than some threshold */
 445         if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
 446             tp->snd_cwnd >= hystart_low_window)
 447                 hystart_update(sk, delay);
 448 }
 449
 450 static struct tcp_congestion_ops cubictcp __read_mostly = {
 451         .init           = bictcp_init,
 452         .ssthresh       = bictcp_recalc_ssthresh,
 453         .cong_avoid     = bictcp_cong_avoid,
 454         .set_state      = bictcp_state,
 455         .undo_cwnd      = bictcp_undo_cwnd,
 456         .pkts_acked     = bictcp_acked,
 457         .owner          = THIS_MODULE,
 458         .name           = "cubic",
 459 };
 460
 461 static int __init cubictcp_register(void)
 462 {
 463         BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 464
 465         /* Precompute a bunch of the scaling factors that are used per-packet
 466          * based on SRTT of 100ms
 467          */
 468
 469         beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
 470                 / (BICTCP_BETA_SCALE - beta);
 471
 472         cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 473
 474         /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 475          *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 476          * the unit of K is bictcp_HZ=2^10, not HZ
 477          *
 478          *  c = bic_scale >> 10
 479          *  rtt = 100ms
 480          *
 481          * the following code has been designed and tested for
 482          * cwnd < 1 million packets
 483          * RTT < 100 seconds
 484          * HZ < 1,000,00  (corresponding to 10 nano-second)
 485          */
 486
 487         /* 1/c * 2^2*bictcp_HZ * srtt */
 488         cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 489
 490         /* divide by bic_scale and by constant Srtt (100ms) */
 491         do_div(cube_factor, bic_scale * 10);
 492
 493         return tcp_register_congestion_control(&cubictcp);
 494 }
 495
 496 static void __exit cubictcp_unregister(void)
 497 {
 498         tcp_unregister_congestion_control(&cubictcp);
 499 }
 500
 501 module_init(cubictcp_register);
 502 module_exit(cubictcp_unregister);
 503
 504 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 505 MODULE_LICENSE("GPL");
 506 MODULE_DESCRIPTION("CUBIC TCP");
 507 MODULE_VERSION("2.3");