linux/net/ipv4/tcp_cubic.c
<<
>>
Prefs
   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       (2U<<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;
  55
  56static u32 cube_rtt_scale __read_mostly;
  57static u32 beta_scale __read_mostly;
  58static u64 cube_factor __read_mostly;
  59
  60/* Note parameters that are used for precomputing scale factors are read-only */
  61module_param(fast_convergence, int, 0644);
  62MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  63module_param(beta, int, 0644);
  64MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  65module_param(initial_ssthresh, int, 0644);
  66MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  67module_param(bic_scale, int, 0444);
  68MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  69module_param(tcp_friendliness, int, 0644);
  70MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  71module_param(hystart, int, 0644);
  72MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
  73module_param(hystart_detect, int, 0644);
  74MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
  75                 " 1: packet-train 2: delay 3: both packet-train and delay");
  76module_param(hystart_low_window, int, 0644);
  77MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
  78
  79/* BIC TCP Parameters */
  80struct bictcp {
  81        u32     cnt;            /* increase cwnd by 1 after ACKs */
  82        u32     last_max_cwnd;  /* last maximum snd_cwnd */
  83        u32     loss_cwnd;      /* congestion window at last loss */
  84        u32     last_cwnd;      /* the last snd_cwnd */
  85        u32     last_time;      /* time when updated last_cwnd */
  86        u32     bic_origin_point;/* origin point of bic function */
  87        u32     bic_K;          /* time to origin point from the beginning of the current epoch */
  88        u32     delay_min;      /* min delay */
  89        u32     epoch_start;    /* beginning of an epoch */
  90        u32     ack_cnt;        /* number of acks */
  91        u32     tcp_cwnd;       /* estimated tcp cwnd */
  92#define ACK_RATIO_SHIFT 4
  93        u16     delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
  94        u8      sample_cnt;     /* number of samples to decide curr_rtt */
  95        u8      found;          /* the exit point is found? */
  96        u32     round_start;    /* beginning of each round */
  97        u32     end_seq;        /* end_seq of the round */
  98        u32     last_jiffies;   /* last time when the ACK spacing is close */
  99        u32     curr_rtt;       /* the minimum rtt of current round */
 100};
 101
 102static inline void bictcp_reset(struct bictcp *ca)
 103{
 104        ca->cnt = 0;
 105        ca->last_max_cwnd = 0;
 106        ca->loss_cwnd = 0;
 107        ca->last_cwnd = 0;
 108        ca->last_time = 0;
 109        ca->bic_origin_point = 0;
 110        ca->bic_K = 0;
 111        ca->delay_min = 0;
 112        ca->epoch_start = 0;
 113        ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
 114        ca->ack_cnt = 0;
 115        ca->tcp_cwnd = 0;
 116        ca->found = 0;
 117}
 118
 119static inline void bictcp_hystart_reset(struct sock *sk)
 120{
 121        struct tcp_sock *tp = tcp_sk(sk);
 122        struct bictcp *ca = inet_csk_ca(sk);
 123
 124        ca->round_start = ca->last_jiffies = jiffies;
 125        ca->end_seq = tp->snd_nxt;
 126        ca->curr_rtt = 0;
 127        ca->sample_cnt = 0;
 128}
 129
 130static void bictcp_init(struct sock *sk)
 131{
 132        bictcp_reset(inet_csk_ca(sk));
 133
 134        if (hystart)
 135                bictcp_hystart_reset(sk);
 136
 137        if (!hystart && initial_ssthresh)
 138                tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 139}
 140
 141/* calculate the cubic root of x using a table lookup followed by one
 142 * Newton-Raphson iteration.
 143 * Avg err ~= 0.195%
 144 */
 145static u32 cubic_root(u64 a)
 146{
 147        u32 x, b, shift;
 148        /*
 149         * cbrt(x) MSB values for x MSB values in [0..63].
 150         * Precomputed then refined by hand - Willy Tarreau
 151         *
 152         * For x in [0..63],
 153         *   v = cbrt(x << 18) - 1
 154         *   cbrt(x) = (v[x] + 10) >> 6
 155         */
 156        static const u8 v[] = {
 157                /* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
 158                /* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
 159                /* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
 160                /* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
 161                /* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
 162                /* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
 163                /* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
 164                /* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
 165        };
 166
 167        b = fls64(a);
 168        if (b < 7) {
 169                /* a in [0..63] */
 170                return ((u32)v[(u32)a] + 35) >> 6;
 171        }
 172
 173        b = ((b * 84) >> 8) - 1;
 174        shift = (a >> (b * 3));
 175
 176        x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
 177
 178        /*
 179         * Newton-Raphson iteration
 180         *                         2
 181         * x    = ( 2 * x  +  a / x  ) / 3
 182         *  k+1          k         k
 183         */
 184        x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
 185        x = ((x * 341) >> 10);
 186        return x;
 187}
 188
 189/*
 190 * Compute congestion window to use.
 191 */
 192static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 193{
 194        u64 offs;
 195        u32 delta, t, bic_target, max_cnt;
 196
 197        ca->ack_cnt++;  /* count the number of ACKs */
 198
 199        if (ca->last_cwnd == cwnd &&
 200            (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
 201                return;
 202
 203        ca->last_cwnd = cwnd;
 204        ca->last_time = tcp_time_stamp;
 205
 206        if (ca->epoch_start == 0) {
 207                ca->epoch_start = tcp_time_stamp;       /* record the beginning of an epoch */
 208                ca->ack_cnt = 1;                        /* start counting */
 209                ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 210
 211                if (ca->last_max_cwnd <= cwnd) {
 212                        ca->bic_K = 0;
 213                        ca->bic_origin_point = cwnd;
 214                } else {
 215                        /* Compute new K based on
 216                         * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 217                         */
 218                        ca->bic_K = cubic_root(cube_factor
 219                                               * (ca->last_max_cwnd - cwnd));
 220                        ca->bic_origin_point = ca->last_max_cwnd;
 221                }
 222        }
 223
 224        /* cubic function - calc*/
 225        /* calculate c * time^3 / rtt,
 226         *  while considering overflow in calculation of time^3
 227         * (so time^3 is done by using 64 bit)
 228         * and without the support of division of 64bit numbers
 229         * (so all divisions are done by using 32 bit)
 230         *  also NOTE the unit of those veriables
 231         *        time  = (t - K) / 2^bictcp_HZ
 232         *        c = bic_scale >> 10
 233         * rtt  = (srtt >> 3) / HZ
 234         * !!! The following code does not have overflow problems,
 235         * if the cwnd < 1 million packets !!!
 236         */
 237
 238        /* change the unit from HZ to bictcp_HZ */
 239        t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
 240             << BICTCP_HZ) / HZ;
 241
 242        if (t < ca->bic_K)              /* t - K */
 243                offs = ca->bic_K - t;
 244        else
 245                offs = t - ca->bic_K;
 246
 247        /* c/rtt * (t-K)^3 */
 248        delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 249        if (t < ca->bic_K)                                      /* below origin*/
 250                bic_target = ca->bic_origin_point - delta;
 251        else                                                    /* above origin*/
 252                bic_target = ca->bic_origin_point + delta;
 253
 254        /* cubic function - calc bictcp_cnt*/
 255        if (bic_target > cwnd) {
 256                ca->cnt = cwnd / (bic_target - cwnd);
 257        } else {
 258                ca->cnt = 100 * cwnd;              /* very small increment*/
 259        }
 260
 261        /* TCP Friendly */
 262        if (tcp_friendliness) {
 263                u32 scale = beta_scale;
 264                delta = (cwnd * scale) >> 3;
 265                while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 266                        ca->ack_cnt -= delta;
 267                        ca->tcp_cwnd++;
 268                }
 269
 270                if (ca->tcp_cwnd > cwnd){       /* if bic is slower than tcp */
 271                        delta = ca->tcp_cwnd - cwnd;
 272                        max_cnt = cwnd / delta;
 273                        if (ca->cnt > max_cnt)
 274                                ca->cnt = max_cnt;
 275                }
 276        }
 277
 278        ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
 279        if (ca->cnt == 0)                       /* cannot be zero */
 280                ca->cnt = 1;
 281}
 282
 283static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 284{
 285        struct tcp_sock *tp = tcp_sk(sk);
 286        struct bictcp *ca = inet_csk_ca(sk);
 287
 288        if (!tcp_is_cwnd_limited(sk, in_flight))
 289                return;
 290
 291        if (tp->snd_cwnd <= tp->snd_ssthresh) {
 292                if (hystart && after(ack, ca->end_seq))
 293                        bictcp_hystart_reset(sk);
 294                tcp_slow_start(tp);
 295        } else {
 296                bictcp_update(ca, tp->snd_cwnd);
 297                tcp_cong_avoid_ai(tp, ca->cnt);
 298        }
 299
 300}
 301
 302static u32 bictcp_recalc_ssthresh(struct sock *sk)
 303{
 304        const struct tcp_sock *tp = tcp_sk(sk);
 305        struct bictcp *ca = inet_csk_ca(sk);
 306
 307        ca->epoch_start = 0;    /* end of epoch */
 308
 309        /* Wmax and fast convergence */
 310        if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 311                ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 312                        / (2 * BICTCP_BETA_SCALE);
 313        else
 314                ca->last_max_cwnd = tp->snd_cwnd;
 315
 316        ca->loss_cwnd = tp->snd_cwnd;
 317
 318        return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 319}
 320
 321static u32 bictcp_undo_cwnd(struct sock *sk)
 322{
 323        struct bictcp *ca = inet_csk_ca(sk);
 324
 325        return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
 326}
 327
 328static void bictcp_state(struct sock *sk, u8 new_state)
 329{
 330        if (new_state == TCP_CA_Loss) {
 331                bictcp_reset(inet_csk_ca(sk));
 332                bictcp_hystart_reset(sk);
 333        }
 334}
 335
 336static void hystart_update(struct sock *sk, u32 delay)
 337{
 338        struct tcp_sock *tp = tcp_sk(sk);
 339        struct bictcp *ca = inet_csk_ca(sk);
 340
 341        if (!(ca->found & hystart_detect)) {
 342                u32 curr_jiffies = jiffies;
 343
 344                /* first detection parameter - ack-train detection */
 345                if (curr_jiffies - ca->last_jiffies <= msecs_to_jiffies(2)) {
 346                        ca->last_jiffies = curr_jiffies;
 347                        if (curr_jiffies - ca->round_start >= ca->delay_min>>4)
 348                                ca->found |= HYSTART_ACK_TRAIN;
 349                }
 350
 351                /* obtain the minimum delay of more than sampling packets */
 352                if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
 353                        if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
 354                                ca->curr_rtt = delay;
 355
 356                        ca->sample_cnt++;
 357                } else {
 358                        if (ca->curr_rtt > ca->delay_min +
 359                            HYSTART_DELAY_THRESH(ca->delay_min>>4))
 360                                ca->found |= HYSTART_DELAY;
 361                }
 362                /*
 363                 * Either one of two conditions are met,
 364                 * we exit from slow start immediately.
 365                 */
 366                if (ca->found & hystart_detect)
 367                        tp->snd_ssthresh = tp->snd_cwnd;
 368        }
 369}
 370
 371/* Track delayed acknowledgment ratio using sliding window
 372 * ratio = (15*ratio + sample) / 16
 373 */
 374static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 375{
 376        const struct inet_connection_sock *icsk = inet_csk(sk);
 377        const struct tcp_sock *tp = tcp_sk(sk);
 378        struct bictcp *ca = inet_csk_ca(sk);
 379        u32 delay;
 380
 381        if (icsk->icsk_ca_state == TCP_CA_Open) {
 382                cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
 383                ca->delayed_ack += cnt;
 384        }
 385
 386        /* Some calls are for duplicates without timetamps */
 387        if (rtt_us < 0)
 388                return;
 389
 390        /* Discard delay samples right after fast recovery */
 391        if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 392                return;
 393
 394        delay = usecs_to_jiffies(rtt_us) << 3;
 395        if (delay == 0)
 396                delay = 1;
 397
 398        /* first time call or link delay decreases */
 399        if (ca->delay_min == 0 || ca->delay_min > delay)
 400                ca->delay_min = delay;
 401
 402        /* hystart triggers when cwnd is larger than some threshold */
 403        if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
 404            tp->snd_cwnd >= hystart_low_window)
 405                hystart_update(sk, delay);
 406}
 407
 408static struct tcp_congestion_ops cubictcp = {
 409        .init           = bictcp_init,
 410        .ssthresh       = bictcp_recalc_ssthresh,
 411        .cong_avoid     = bictcp_cong_avoid,
 412        .set_state      = bictcp_state,
 413        .undo_cwnd      = bictcp_undo_cwnd,
 414        .pkts_acked     = bictcp_acked,
 415        .owner          = THIS_MODULE,
 416        .name           = "cubic",
 417};
 418
 419static int __init cubictcp_register(void)
 420{
 421        BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 422
 423        /* Precompute a bunch of the scaling factors that are used per-packet
 424         * based on SRTT of 100ms
 425         */
 426
 427        beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
 428
 429        cube_rtt_scale = (bic_scale * 10);      /* 1024*c/rtt */
 430
 431        /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 432         *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 433         * the unit of K is bictcp_HZ=2^10, not HZ
 434         *
 435         *  c = bic_scale >> 10
 436         *  rtt = 100ms
 437         *
 438         * the following code has been designed and tested for
 439         * cwnd < 1 million packets
 440         * RTT < 100 seconds
 441         * HZ < 1,000,00  (corresponding to 10 nano-second)
 442         */
 443
 444        /* 1/c * 2^2*bictcp_HZ * srtt */
 445        cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 446
 447        /* divide by bic_scale and by constant Srtt (100ms) */
 448        do_div(cube_factor, bic_scale * 10);
 449
 450        return tcp_register_congestion_control(&cubictcp);
 451}
 452
 453static void __exit cubictcp_unregister(void)
 454{
 455        tcp_unregister_congestion_control(&cubictcp);
 456}
 457
 458module_init(cubictcp_register);
 459module_exit(cubictcp_unregister);
 460
 461MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 462MODULE_LICENSE("GPL");
 463MODULE_DESCRIPTION("CUBIC TCP");
 464MODULE_VERSION("2.3");
 465