net/ipv4/tcp_cubic.c

   1 /*
   2  * TCP CUBIC: Binary Increase Congestion control for TCP v2.0
   3  *
   4  * This is from the implementation of CUBIC TCP in
   5  * Injong Rhee, Lisong Xu.
   6  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant
   7  *  in PFLDnet 2005
   8  * Available from:
   9  *  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
  10  *
  11  * Unless CUBIC is enabled and congestion window is large
  12  * this behaves the same as the original Reno.
  13  */
  14
  15 #include <linux/config.h>
  16 #include <linux/mm.h>
  17 #include <linux/module.h>
  18 #include <net/tcp.h>
  19 #include <asm/div64.h>
  20
  21 #define BICTCP_BETA_SCALE    1024       /* Scale factor beta calculation
  22                                          * max_cwnd = snd_cwnd * beta
  23                                          */
  24 #define BICTCP_B                4        /*
  25                                           * In binary search,
  26                                           * go to point (max+min)/N
  27                                           */
  28 #define BICTCP_HZ               10      /* BIC HZ 2^10 = 1024 */
  29
  30 static int fast_convergence = 1;
  31 static int max_increment = 16;
  32 static int beta = 819;          /* = 819/1024 (BICTCP_BETA_SCALE) */
  33 static int initial_ssthresh = 100;
  34 static int bic_scale = 41;
  35 static int tcp_friendliness = 1;
  36
  37 static u32 cube_rtt_scale;
  38 static u32 beta_scale;
  39 static u64 cube_factor;
  40
  41 /* Note parameters that are used for precomputing scale factors are read-only */
  42 module_param(fast_convergence, int, 0644);
  43 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
  44 module_param(max_increment, int, 0644);
  45 MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
  46 module_param(beta, int, 0444);
  47 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
  48 module_param(initial_ssthresh, int, 0644);
  49 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
  50 module_param(bic_scale, int, 0444);
  51 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
  52 module_param(tcp_friendliness, int, 0644);
  53 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
  54
  55
  56 /* BIC TCP Parameters */
  57 struct bictcp {
  58         u32     cnt;            /* increase cwnd by 1 after ACKs */
  59         u32     last_max_cwnd;  /* last maximum snd_cwnd */
  60         u32     loss_cwnd;      /* congestion window at last loss */
  61         u32     last_cwnd;      /* the last snd_cwnd */
  62         u32     last_time;      /* time when updated last_cwnd */
  63         u32     bic_origin_point;/* origin point of bic function */
  64         u32     bic_K;          /* time to origin point from the beginning of the current epoch */
  65         u32     delay_min;      /* min delay */
  66         u32     epoch_start;    /* beginning of an epoch */
  67         u32     ack_cnt;        /* number of acks */
  68         u32     tcp_cwnd;       /* estimated tcp cwnd */
  69 #define ACK_RATIO_SHIFT 4
  70         u32     delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
  71 };
  72
  73 static inline void bictcp_reset(struct bictcp *ca)
  74 {
  75         ca->cnt = 0;
  76         ca->last_max_cwnd = 0;
  77         ca->loss_cwnd = 0;
  78         ca->last_cwnd = 0;
  79         ca->last_time = 0;
  80         ca->bic_origin_point = 0;
  81         ca->bic_K = 0;
  82         ca->delay_min = 0;
  83         ca->epoch_start = 0;
  84         ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
  85         ca->ack_cnt = 0;
  86         ca->tcp_cwnd = 0;
  87 }
  88
  89 static void bictcp_init(struct sock *sk)
  90 {
  91         bictcp_reset(inet_csk_ca(sk));
  92         if (initial_ssthresh)
  93                 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
  94 }
  95
  96 /* 65536 times the cubic root */
  97 static const u64 cubic_table[8]
  98         = {0, 65536, 82570, 94519, 104030, 112063, 119087, 125367};
  99
 100 /*
 101  * calculate the cubic root of x
 102  * the basic idea is that x can be expressed as i*8^j
 103  * so cubic_root(x) = cubic_root(i)*2^j
 104  *  in the following code, x is i, and y is 2^j
 105  *  because of integer calculation, there are errors in calculation
 106  *  so finally use binary search to find out the exact solution
 107  */
 108 static u32 cubic_root(u64 x)
 109 {
 110         u64 y, app, target, start, end, mid, start_diff, end_diff;
 111
 112         if (x == 0)
 113                 return 0;
 114
 115         target = x;
 116
 117         /* first estimate lower and upper bound */
 118         y = 1;
 119         while (x >= 8){
 120                 x = (x >> 3);
 121                 y = (y << 1);
 122         }
 123         start = (y*cubic_table[x])>>16;
 124         if (x==7)
 125                 end = (y<<1);
 126         else
 127                 end = (y*cubic_table[x+1]+65535)>>16;
 128
 129         /* binary search for more accurate one */
 130         while (start < end-1) {
 131                 mid = (start+end) >> 1;
 132                 app = mid*mid*mid;
 133                 if (app < target)
 134                         start = mid;
 135                 else if (app > target)
 136                         end = mid;
 137                 else
 138                         return mid;
 139         }
 140
 141         /* find the most accurate one from start and end */
 142         app = start*start*start;
 143         if (app < target)
 144                 start_diff = target - app;
 145         else
 146                 start_diff = app - target;
 147         app = end*end*end;
 148         if (app < target)
 149                 end_diff = target - app;
 150         else
 151                 end_diff = app - target;
 152
 153         if (start_diff < end_diff)
 154                 return (u32)start;
 155         else
 156                 return (u32)end;
 157 }
 158
 159 /*
 160  * Compute congestion window to use.
 161  */
 162 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 163 {
 164         u64 offs;
 165         u32 delta, t, bic_target, min_cnt, max_cnt;
 166
 167         ca->ack_cnt++;  /* count the number of ACKs */
 168
 169         if (ca->last_cwnd == cwnd &&
 170             (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
 171                 return;
 172
 173         ca->last_cwnd = cwnd;
 174         ca->last_time = tcp_time_stamp;
 175
 176         if (ca->epoch_start == 0) {
 177                 ca->epoch_start = tcp_time_stamp;       /* record the beginning of an epoch */
 178                 ca->ack_cnt = 1;                        /* start counting */
 179                 ca->tcp_cwnd = cwnd;                    /* syn with cubic */
 180
 181                 if (ca->last_max_cwnd <= cwnd) {
 182                         ca->bic_K = 0;
 183                         ca->bic_origin_point = cwnd;
 184                 } else {
 185                         /* Compute new K based on
 186                          * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
 187                          */
 188                         ca->bic_K = cubic_root(cube_factor
 189                                                * (ca->last_max_cwnd - cwnd));
 190                         ca->bic_origin_point = ca->last_max_cwnd;
 191                 }
 192         }
 193
 194         /* cubic function - calc*/
 195         /* calculate c * time^3 / rtt,
 196          *  while considering overflow in calculation of time^3
 197          * (so time^3 is done by using 64 bit)
 198          * and without the support of division of 64bit numbers
 199          * (so all divisions are done by using 32 bit)
 200          *  also NOTE the unit of those veriables
 201          *        time  = (t - K) / 2^bictcp_HZ
 202          *        c = bic_scale >> 10
 203          * rtt  = (srtt >> 3) / HZ
 204          * !!! The following code does not have overflow problems,
 205          * if the cwnd < 1 million packets !!!
 206          */
 207
 208         /* change the unit from HZ to bictcp_HZ */
 209         t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
 210              << BICTCP_HZ) / HZ;
 211
 212         if (t < ca->bic_K)              /* t - K */
 213                 offs = ca->bic_K - t;
 214         else
 215                 offs = t - ca->bic_K;
 216
 217         /* c/rtt * (t-K)^3 */
 218         delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
 219         if (t < ca->bic_K)                                      /* below origin*/
 220                 bic_target = ca->bic_origin_point - delta;
 221         else                                                    /* above origin*/
 222                 bic_target = ca->bic_origin_point + delta;
 223
 224         /* cubic function - calc bictcp_cnt*/
 225         if (bic_target > cwnd) {
 226                 ca->cnt = cwnd / (bic_target - cwnd);
 227         } else {
 228                 ca->cnt = 100 * cwnd;              /* very small increment*/
 229         }
 230
 231         if (ca->delay_min > 0) {
 232                 /* max increment = Smax * rtt / 0.1  */
 233                 min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min);
 234                 if (ca->cnt < min_cnt)
 235                         ca->cnt = min_cnt;
 236         }
 237
 238         /* slow start and low utilization  */
 239         if (ca->loss_cwnd == 0)         /* could be aggressive in slow start */
 240                 ca->cnt = 50;
 241
 242         /* TCP Friendly */
 243         if (tcp_friendliness) {
 244                 u32 scale = beta_scale;
 245                 delta = (cwnd * scale) >> 3;
 246                 while (ca->ack_cnt > delta) {           /* update tcp cwnd */
 247                         ca->ack_cnt -= delta;
 248                         ca->tcp_cwnd++;
 249                 }
 250
 251                 if (ca->tcp_cwnd > cwnd){       /* if bic is slower than tcp */
 252                         delta = ca->tcp_cwnd - cwnd;
 253                         max_cnt = cwnd / delta;
 254                         if (ca->cnt > max_cnt)
 255                                 ca->cnt = max_cnt;
 256                 }
 257         }
 258
 259         ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
 260         if (ca->cnt == 0)                       /* cannot be zero */
 261                 ca->cnt = 1;
 262 }
 263
 264
 265 /* Keep track of minimum rtt */
 266 static inline void measure_delay(struct sock *sk)
 267 {
 268         const struct tcp_sock *tp = tcp_sk(sk);
 269         struct bictcp *ca = inet_csk_ca(sk);
 270         u32 delay;
 271
 272         /* No time stamp */
 273         if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) ||
 274              /* Discard delay samples right after fast recovery */
 275             (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
 276                 return;
 277
 278         delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
 279         if (delay == 0)
 280                 delay = 1;
 281
 282         /* first time call or link delay decreases */
 283         if (ca->delay_min == 0 || ca->delay_min > delay)
 284                 ca->delay_min = delay;
 285 }
 286
 287 static void bictcp_cong_avoid(struct sock *sk, u32 ack,
 288                               u32 seq_rtt, u32 in_flight, int data_acked)
 289 {
 290         struct tcp_sock *tp = tcp_sk(sk);
 291         struct bictcp *ca = inet_csk_ca(sk);
 292
 293         if (data_acked)
 294                 measure_delay(sk);
 295
 296         if (!tcp_is_cwnd_limited(sk, in_flight))
 297                 return;
 298
 299         if (tp->snd_cwnd <= tp->snd_ssthresh)
 300                 tcp_slow_start(tp);
 301         else {
 302                 bictcp_update(ca, tp->snd_cwnd);
 303
 304                 /* In dangerous area, increase slowly.
 305                  * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
 306                  */
 307                 if (tp->snd_cwnd_cnt >= ca->cnt) {
 308                         if (tp->snd_cwnd < tp->snd_cwnd_clamp)
 309                                 tp->snd_cwnd++;
 310                         tp->snd_cwnd_cnt = 0;
 311                 } else
 312                         tp->snd_cwnd_cnt++;
 313         }
 314
 315 }
 316
 317 static u32 bictcp_recalc_ssthresh(struct sock *sk)
 318 {
 319         const struct tcp_sock *tp = tcp_sk(sk);
 320         struct bictcp *ca = inet_csk_ca(sk);
 321
 322         ca->epoch_start = 0;    /* end of epoch */
 323
 324         /* Wmax and fast convergence */
 325         if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
 326                 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
 327                         / (2 * BICTCP_BETA_SCALE);
 328         else
 329                 ca->last_max_cwnd = tp->snd_cwnd;
 330
 331         ca->loss_cwnd = tp->snd_cwnd;
 332
 333         return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
 334 }
 335
 336 static u32 bictcp_undo_cwnd(struct sock *sk)
 337 {
 338         struct bictcp *ca = inet_csk_ca(sk);
 339
 340         return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
 341 }
 342
 343 static u32 bictcp_min_cwnd(struct sock *sk)
 344 {
 345         return tcp_sk(sk)->snd_ssthresh;
 346 }
 347
 348 static void bictcp_state(struct sock *sk, u8 new_state)
 349 {
 350         if (new_state == TCP_CA_Loss)
 351                 bictcp_reset(inet_csk_ca(sk));
 352 }
 353
 354 /* Track delayed acknowledgment ratio using sliding window
 355  * ratio = (15*ratio + sample) / 16
 356  */
 357 static void bictcp_acked(struct sock *sk, u32 cnt)
 358 {
 359         const struct inet_connection_sock *icsk = inet_csk(sk);
 360
 361         if (cnt > 0 && icsk->icsk_ca_state == TCP_CA_Open) {
 362                 struct bictcp *ca = inet_csk_ca(sk);
 363                 cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
 364                 ca->delayed_ack += cnt;
 365         }
 366 }
 367
 368
 369 static struct tcp_congestion_ops cubictcp = {
 370         .init           = bictcp_init,
 371         .ssthresh       = bictcp_recalc_ssthresh,
 372         .cong_avoid     = bictcp_cong_avoid,
 373         .set_state      = bictcp_state,
 374         .undo_cwnd      = bictcp_undo_cwnd,
 375         .min_cwnd       = bictcp_min_cwnd,
 376         .pkts_acked     = bictcp_acked,
 377         .owner          = THIS_MODULE,
 378         .name           = "cubic",
 379 };
 380
 381 static int __init cubictcp_register(void)
 382 {
 383         BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
 384
 385         /* Precompute a bunch of the scaling factors that are used per-packet
 386          * based on SRTT of 100ms
 387          */
 388
 389         beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
 390
 391         cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */
 392
 393         /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
 394          *  so K = cubic_root( (wmax-cwnd)*rtt/c )
 395          * the unit of K is bictcp_HZ=2^10, not HZ
 396          *
 397          *  c = bic_scale >> 10
 398          *  rtt = 100ms
 399          *
 400          * the following code has been designed and tested for
 401          * cwnd < 1 million packets
 402          * RTT < 100 seconds
 403          * HZ < 1,000,00  (corresponding to 10 nano-second)
 404          */
 405
 406         /* 1/c * 2^2*bictcp_HZ * srtt */
 407         cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
 408
 409         /* divide by bic_scale and by constant Srtt (100ms) */
 410         do_div(cube_factor, bic_scale * 10);
 411
 412         return tcp_register_congestion_control(&cubictcp);
 413 }
 414
 415 static void __exit cubictcp_unregister(void)
 416 {
 417         tcp_unregister_congestion_control(&cubictcp);
 418 }
 419
 420 module_init(cubictcp_register);
 421 module_exit(cubictcp_unregister);
 422
 423 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 424 MODULE_LICENSE("GPL");
 425 MODULE_DESCRIPTION("CUBIC TCP");
 426 MODULE_VERSION("2.0");