linux/net/ipv4/tcp_cubic.c
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   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
  46static int fast_convergence __read_mostly = 1;
  47static int beta __read_mostly = 717;    /* = 717/1024 (BICTCP_BETA_SCALE) */
  48static int initial_ssthresh __read_mostly;
  49static int bic_scale __read_mostly = 41;
  50static int tcp_friendliness __read_mostly = 1;
  51
  52static int hystart __read_mostly = 1;
  53static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
  54static int hystart_low_window __read_mostly = 16;
  55static int hystart_ack_delta __read_mostly = 2;
  56
  57static u32 cube_rtt_scale __read_mostly;
  58static u32 beta_scale __read_mostly;
  59static u64 cube_factor __read_mostly;
  60
  61/* Note parameters that are used for precomputing scale factors are read-only */
  62module_param(fast_convergence, int, 0644);
  63MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  64module_param(beta, int, 0644);
  65MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  66module_param(initial_ssthresh, int, 0644);
  67MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  68module_param(bic_scale, int, 0444);
  69MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  70module_param(tcp_friendliness, int, 0644);
  71MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  72module_param(hystart, int, 0644);
  73MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
  74module_param(hystart_detect, int, 0644);
  75MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
  76                 " 1: packet-train 2: delay 3: both packet-train and delay");
  77module_param(hystart_low_window, int, 0644);
  78MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
  79module_param(hystart_ack_delta, int, 0644);
  80MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
  81
  82/* BIC TCP Parameters */
  83struct 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
 105static 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
 120static 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
 129static 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
 140static 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/* calculate the cubic root of x using a table lookup followed by one
 155 * Newton-Raphson iteration.
 156 * Avg err ~= 0.195%
 157 */
 158static u32 cubic_root(u64 a)
 159{
 160        u32 x, b, shift;
 161        /*
 162         * cbrt(x) MSB values for x MSB values in [0..63].
 163         * Precomputed then refined by hand - Willy Tarreau
 164         *
 165         * For x in [0..63],
 166         *   v = cbrt(x << 18) - 1
 167         *   cbrt(x) = (v[x] + 10) >> 6
 168         */
 169        static const u8 v[] = {
 170                /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
 171                /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
 172                /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
 173                /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
 174                /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
 175                /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
 176                /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
 177                /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
 178        };
 179
 180        b = fls64(a);
 181        if (b < 7) {
 182                /* a in [0..63] */
 183                return ((u32)v[(u32)a] + 35) >> 6;
 184        }
 185
 186        b = ((b * 84) >> 8) - 1;
 187        shift = (a >> (b * 3));
 188
 189        x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
 190
 191        /*
 192         * Newton-Raphson iteration
 193         *                         2
 194         * x    = ( 2 * x  +  a / x  ) / 3
 195         *  k+1          k         k
 196         */
 197        x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
 198        x = ((x * 341) >> 10);
 199        return x;
 200}
 201
 202/*
 203 * Compute congestion window to use.
 204 */
 205static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
 206{
 207        u32 delta, bic_target, max_cnt;
 208        u64 offs, t;
 209
 210        ca->ack_cnt += acked;   /* count the number of ACKed packets */
 211
 212        if (ca->last_cwnd == cwnd &&
 213            (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
 214                return;
 215
 216        /* The CUBIC function can update ca->cnt at most once per jiffy.
 217         * On all cwnd reduction events, ca->epoch_start is set to 0,
 218         * which will force a recalculation of ca->cnt.
 219         */
 220        if (ca->epoch_start && tcp_time_stamp == ca->last_time)
 221                goto tcp_friendliness;
 222
 223        ca->last_cwnd = cwnd;
 224        ca->last_time = tcp_time_stamp;
 225
 226        if (ca->epoch_start == 0) {
 227                ca->epoch_start = tcp_time_stamp;       /* record beginning */
 228                ca->ack_cnt = acked;                    /* start counting */
 229                ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 230
 231                if (ca->last_max_cwnd <= cwnd) {
 232                        ca->bic_K = 0;
 233                        ca->bic_origin_point = cwnd;
 234                } else {
 235                        /* Compute new K based on
 236                         * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 237                         */
 238                        ca->bic_K = cubic_root(cube_factor
 239                                               * (ca->last_max_cwnd - cwnd));
 240                        ca->bic_origin_point = ca->last_max_cwnd;
 241                }
 242        }
 243
 244        /* cubic function - calc*/
 245        /* calculate c * time^3 / rtt,
 246         *  while considering overflow in calculation of time^3
 247         * (so time^3 is done by using 64 bit)
 248         * and without the support of division of 64bit numbers
 249         * (so all divisions are done by using 32 bit)
 250         *  also NOTE the unit of those veriables
 251         *        time  = (t - K) / 2^bictcp_HZ
 252         *        c = bic_scale >> 10
 253         * rtt  = (srtt >> 3) / HZ
 254         * !!! The following code does not have overflow problems,
 255         * if the cwnd < 1 million packets !!!
 256         */
 257
 258        t = (s32)(tcp_time_stamp - ca->epoch_start);
 259        t += msecs_to_jiffies(ca->delay_min >> 3);
 260        /* change the unit from HZ to bictcp_HZ */
 261        t <<= BICTCP_HZ;
 262        do_div(t, HZ);
 263
 264        if (t < ca->bic_K)              /* t - K */
 265                offs = ca->bic_K - t;
 266        else
 267                offs = t - ca->bic_K;
 268
 269        /* c/rtt * (t-K)^3 */
 270        delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 271        if (t < ca->bic_K)                            /* below origin*/
 272                bic_target = ca->bic_origin_point - delta;
 273        else                                          /* above origin*/
 274                bic_target = ca->bic_origin_point + delta;
 275
 276        /* cubic function - calc bictcp_cnt*/
 277        if (bic_target > cwnd) {
 278                ca->cnt = cwnd / (bic_target - cwnd);
 279        } else {
 280                ca->cnt = 100 * cwnd;              /* very small increment*/
 281        }
 282
 283        /*
 284         * The initial growth of cubic function may be too conservative
 285         * when the available bandwidth is still unknown.
 286         */
 287        if (ca->last_max_cwnd == 0 && ca->cnt > 20)
 288                ca->cnt = 20;   /* increase cwnd 5% per RTT */
 289
 290tcp_friendliness:
 291        /* TCP Friendly */
 292        if (tcp_friendliness) {
 293                u32 scale = beta_scale;
 294
 295                delta = (cwnd * scale) >> 3;
 296                while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 297                        ca->ack_cnt -= delta;
 298                        ca->tcp_cwnd++;
 299                }
 300
 301                if (ca->tcp_cwnd > cwnd) {      /* if bic is slower than tcp */
 302                        delta = ca->tcp_cwnd - cwnd;
 303                        max_cnt = cwnd / delta;
 304                        if (ca->cnt > max_cnt)
 305                                ca->cnt = max_cnt;
 306                }
 307        }
 308
 309        if (ca->cnt == 0)                       /* cannot be zero */
 310                ca->cnt = 1;
 311}
 312
 313static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 314{
 315        struct tcp_sock *tp = tcp_sk(sk);
 316        struct bictcp *ca = inet_csk_ca(sk);
 317
 318        if (!tcp_is_cwnd_limited(sk))
 319                return;
 320
 321        if (tp->snd_cwnd <= tp->snd_ssthresh) {
 322                if (hystart && after(ack, ca->end_seq))
 323                        bictcp_hystart_reset(sk);
 324                acked = tcp_slow_start(tp, acked);
 325                if (!acked)
 326                        return;
 327        }
 328        bictcp_update(ca, tp->snd_cwnd, acked);
 329        tcp_cong_avoid_ai(tp, ca->cnt, acked);
 330}
 331
 332static u32 bictcp_recalc_ssthresh(struct sock *sk)
 333{
 334        const struct tcp_sock *tp = tcp_sk(sk);
 335        struct bictcp *ca = inet_csk_ca(sk);
 336
 337        ca->epoch_start = 0;    /* end of epoch */
 338
 339        /* Wmax and fast convergence */
 340        if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 341                ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 342                        / (2 * BICTCP_BETA_SCALE);
 343        else
 344                ca->last_max_cwnd = tp->snd_cwnd;
 345
 346        ca->loss_cwnd = tp->snd_cwnd;
 347
 348        return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 349}
 350
 351static u32 bictcp_undo_cwnd(struct sock *sk)
 352{
 353        struct bictcp *ca = inet_csk_ca(sk);
 354
 355        return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
 356}
 357
 358static void bictcp_state(struct sock *sk, u8 new_state)
 359{
 360        if (new_state == TCP_CA_Loss) {
 361                bictcp_reset(inet_csk_ca(sk));
 362                bictcp_hystart_reset(sk);
 363        }
 364}
 365
 366static void hystart_update(struct sock *sk, u32 delay)
 367{
 368        struct tcp_sock *tp = tcp_sk(sk);
 369        struct bictcp *ca = inet_csk_ca(sk);
 370
 371        if (ca->found & hystart_detect)
 372                return;
 373
 374        if (hystart_detect & HYSTART_ACK_TRAIN) {
 375                u32 now = bictcp_clock();
 376
 377                /* first detection parameter - ack-train detection */
 378                if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
 379                        ca->last_ack = now;
 380                        if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
 381                                ca->found |= HYSTART_ACK_TRAIN;
 382                                NET_INC_STATS_BH(sock_net(sk),
 383                                                 LINUX_MIB_TCPHYSTARTTRAINDETECT);
 384                                NET_ADD_STATS_BH(sock_net(sk),
 385                                                 LINUX_MIB_TCPHYSTARTTRAINCWND,
 386                                                 tp->snd_cwnd);
 387                                tp->snd_ssthresh = tp->snd_cwnd;
 388                        }
 389                }
 390        }
 391
 392        if (hystart_detect & HYSTART_DELAY) {
 393                /* obtain the minimum delay of more than sampling packets */
 394                if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
 395                        if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
 396                                ca->curr_rtt = delay;
 397
 398                        ca->sample_cnt++;
 399                } else {
 400                        if (ca->curr_rtt > ca->delay_min +
 401                            HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
 402                                ca->found |= HYSTART_DELAY;
 403                                NET_INC_STATS_BH(sock_net(sk),
 404                                                 LINUX_MIB_TCPHYSTARTDELAYDETECT);
 405                                NET_ADD_STATS_BH(sock_net(sk),
 406                                                 LINUX_MIB_TCPHYSTARTDELAYCWND,
 407                                                 tp->snd_cwnd);
 408                                tp->snd_ssthresh = tp->snd_cwnd;
 409                        }
 410                }
 411        }
 412}
 413
 414/* Track delayed acknowledgment ratio using sliding window
 415 * ratio = (15*ratio + sample) / 16
 416 */
 417static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 418{
 419        const struct tcp_sock *tp = tcp_sk(sk);
 420        struct bictcp *ca = inet_csk_ca(sk);
 421        u32 delay;
 422
 423        /* Some calls are for duplicates without timetamps */
 424        if (rtt_us < 0)
 425                return;
 426
 427        /* Discard delay samples right after fast recovery */
 428        if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 429                return;
 430
 431        delay = (rtt_us << 3) / USEC_PER_MSEC;
 432        if (delay == 0)
 433                delay = 1;
 434
 435        /* first time call or link delay decreases */
 436        if (ca->delay_min == 0 || ca->delay_min > delay)
 437                ca->delay_min = delay;
 438
 439        /* hystart triggers when cwnd is larger than some threshold */
 440        if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
 441            tp->snd_cwnd >= hystart_low_window)
 442                hystart_update(sk, delay);
 443}
 444
 445static struct tcp_congestion_ops cubictcp __read_mostly = {
 446        .init           = bictcp_init,
 447        .ssthresh       = bictcp_recalc_ssthresh,
 448        .cong_avoid     = bictcp_cong_avoid,
 449        .set_state      = bictcp_state,
 450        .undo_cwnd      = bictcp_undo_cwnd,
 451        .pkts_acked     = bictcp_acked,
 452        .owner          = THIS_MODULE,
 453        .name           = "cubic",
 454};
 455
 456static int __init cubictcp_register(void)
 457{
 458        BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 459
 460        /* Precompute a bunch of the scaling factors that are used per-packet
 461         * based on SRTT of 100ms
 462         */
 463
 464        beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
 465                / (BICTCP_BETA_SCALE - beta);
 466
 467        cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 468
 469        /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 470         *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 471         * the unit of K is bictcp_HZ=2^10, not HZ
 472         *
 473         *  c = bic_scale >> 10
 474         *  rtt = 100ms
 475         *
 476         * the following code has been designed and tested for
 477         * cwnd < 1 million packets
 478         * RTT < 100 seconds
 479         * HZ < 1,000,00  (corresponding to 10 nano-second)
 480         */
 481
 482        /* 1/c * 2^2*bictcp_HZ * srtt */
 483        cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 484
 485        /* divide by bic_scale and by constant Srtt (100ms) */
 486        do_div(cube_factor, bic_scale * 10);
 487
 488        return tcp_register_congestion_control(&cubictcp);
 489}
 490
 491static void __exit cubictcp_unregister(void)
 492{
 493        tcp_unregister_congestion_control(&cubictcp);
 494}
 495
 496module_init(cubictcp_register);
 497module_exit(cubictcp_unregister);
 498
 499MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 500MODULE_LICENSE("GPL");
 501MODULE_DESCRIPTION("CUBIC TCP");
 502MODULE_VERSION("2.3");
 503
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