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
 154static 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_time_stamp;
 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 */
 179static 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 */
 226static 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_time_stamp - 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_time_stamp == ca->last_time)
 242                goto tcp_friendliness;
 243
 244        ca->last_cwnd = cwnd;
 245        ca->last_time = tcp_time_stamp;
 246
 247        if (ca->epoch_start == 0) {
 248                ca->epoch_start = tcp_time_stamp;       /* 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_time_stamp - 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
 311tcp_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
 336static 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
 355static 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
 374static 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
 381static 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
 389static 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_BH(sock_net(sk),
 406                                                 LINUX_MIB_TCPHYSTARTTRAINDETECT);
 407                                NET_ADD_STATS_BH(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_BH(sock_net(sk),
 427                                                 LINUX_MIB_TCPHYSTARTDELAYDETECT);
 428                                NET_ADD_STATS_BH(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 */
 440static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 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 (rtt_us < 0)
 448                return;
 449
 450        /* Discard delay samples right after fast recovery */
 451        if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 452                return;
 453
 454        delay = (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
 468static 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
 480static 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
 515static void __exit cubictcp_unregister(void)
 516{
 517        tcp_unregister_congestion_control(&cubictcp);
 518}
 519
 520module_init(cubictcp_register);
 521module_exit(cubictcp_unregister);
 522
 523MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 524MODULE_LICENSE("GPL");
 525MODULE_DESCRIPTION("CUBIC TCP");
 526MODULE_VERSION("2.3");
 527
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