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, "hybrid 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 last_cwnd; /* the last snd_cwnd */ 87 u32 last_time; /* time when updated last_cwnd */ 88 u32 bic_origin_point;/* origin point of bic function */ 89 u32 bic_K; /* time to origin point 90 from the beginning of the current epoch */ 91 u32 delay_min; /* min delay (msec << 3) */ 92 u32 epoch_start; /* beginning of an epoch */ 93 u32 ack_cnt; /* number of acks */ 94 u32 tcp_cwnd; /* estimated tcp cwnd */ 95 u16 unused; 96 u8 sample_cnt; /* number of samples to decide curr_rtt */ 97 u8 found; /* the exit point is found? */ 98 u32 round_start; /* beginning of each round */ 99 u32 end_seq; /* end_seq of the round */ 100 u32 last_ack; /* last time when the ACK spacing is close */ 101 u32 curr_rtt; /* the minimum rtt of current round */ 102 }; 103 104 static inline void bictcp_reset(struct bictcp *ca) 105 { 106 ca->cnt = 0; 107 ca->last_max_cwnd = 0; 108 ca->last_cwnd = 0; 109 ca->last_time = 0; 110 ca->bic_origin_point = 0; 111 ca->bic_K = 0; 112 ca->delay_min = 0; 113 ca->epoch_start = 0; 114 ca->ack_cnt = 0; 115 ca->tcp_cwnd = 0; 116 ca->found = 0; 117 } 118 119 static inline u32 bictcp_clock(void) 120 { 121 #if HZ < 1000 122 return ktime_to_ms(ktime_get_real()); 123 #else 124 return jiffies_to_msecs(jiffies); 125 #endif 126 } 127 128 static inline void bictcp_hystart_reset(struct sock *sk) 129 { 130 struct tcp_sock *tp = tcp_sk(sk); 131 struct bictcp *ca = inet_csk_ca(sk); 132 133 ca->round_start = ca->last_ack = bictcp_clock(); 134 ca->end_seq = tp->snd_nxt; 135 ca->curr_rtt = 0; 136 ca->sample_cnt = 0; 137 } 138 139 static void bictcp_init(struct sock *sk) 140 { 141 struct bictcp *ca = inet_csk_ca(sk); 142 143 bictcp_reset(ca); 144 145 if (hystart) 146 bictcp_hystart_reset(sk); 147 148 if (!hystart && initial_ssthresh) 149 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 150 } 151 152 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) 153 { 154 if (event == CA_EVENT_TX_START) { 155 struct bictcp *ca = inet_csk_ca(sk); 156 u32 now = tcp_jiffies32; 157 s32 delta; 158 159 delta = now - tcp_sk(sk)->lsndtime; 160 161 /* We were application limited (idle) for a while. 162 * Shift epoch_start to keep cwnd growth to cubic curve. 163 */ 164 if (ca->epoch_start && delta > 0) { 165 ca->epoch_start += delta; 166 if (after(ca->epoch_start, now)) 167 ca->epoch_start = now; 168 } 169 return; 170 } 171 } 172 173 /* calculate the cubic root of x using a table lookup followed by one 174 * Newton-Raphson iteration. 175 * Avg err ~= 0.195% 176 */ 177 static u32 cubic_root(u64 a) 178 { 179 u32 x, b, shift; 180 /* 181 * cbrt(x) MSB values for x MSB values in [0..63]. 182 * Precomputed then refined by hand - Willy Tarreau 183 * 184 * For x in [0..63], 185 * v = cbrt(x << 18) - 1 186 * cbrt(x) = (v[x] + 10) >> 6 187 */ 188 static const u8 v[] = { 189 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 190 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 191 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 192 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 193 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 194 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 195 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 196 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 197 }; 198 199 b = fls64(a); 200 if (b < 7) { 201 /* a in [0..63] */ 202 return ((u32)v[(u32)a] + 35) >> 6; 203 } 204 205 b = ((b * 84) >> 8) - 1; 206 shift = (a >> (b * 3)); 207 208 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 209 210 /* 211 * Newton-Raphson iteration 212 * 2 213 * x = ( 2 * x + a / x ) / 3 214 * k+1 k k 215 */ 216 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); 217 x = ((x * 341) >> 10); 218 return x; 219 } 220 221 /* 222 * Compute congestion window to use. 223 */ 224 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) 225 { 226 u32 delta, bic_target, max_cnt; 227 u64 offs, t; 228 229 ca->ack_cnt += acked; /* count the number of ACKed packets */ 230 231 if (ca->last_cwnd == cwnd && 232 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32) 233 return; 234 235 /* The CUBIC function can update ca->cnt at most once per jiffy. 236 * On all cwnd reduction events, ca->epoch_start is set to 0, 237 * which will force a recalculation of ca->cnt. 238 */ 239 if (ca->epoch_start && tcp_jiffies32 == ca->last_time) 240 goto tcp_friendliness; 241 242 ca->last_cwnd = cwnd; 243 ca->last_time = tcp_jiffies32; 244 245 if (ca->epoch_start == 0) { 246 ca->epoch_start = tcp_jiffies32; /* record beginning */ 247 ca->ack_cnt = acked; /* start counting */ 248 ca->tcp_cwnd = cwnd; /* syn with cubic */ 249 250 if (ca->last_max_cwnd <= cwnd) { 251 ca->bic_K = 0; 252 ca->bic_origin_point = cwnd; 253 } else { 254 /* Compute new K based on 255 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 256 */ 257 ca->bic_K = cubic_root(cube_factor 258 * (ca->last_max_cwnd - cwnd)); 259 ca->bic_origin_point = ca->last_max_cwnd; 260 } 261 } 262 263 /* cubic function - calc*/ 264 /* calculate c * time^3 / rtt, 265 * while considering overflow in calculation of time^3 266 * (so time^3 is done by using 64 bit) 267 * and without the support of division of 64bit numbers 268 * (so all divisions are done by using 32 bit) 269 * also NOTE the unit of those veriables 270 * time = (t - K) / 2^bictcp_HZ 271 * c = bic_scale >> 10 272 * rtt = (srtt >> 3) / HZ 273 * !!! The following code does not have overflow problems, 274 * if the cwnd < 1 million packets !!! 275 */ 276 277 t = (s32)(tcp_jiffies32 - ca->epoch_start); 278 t += msecs_to_jiffies(ca->delay_min >> 3); 279 /* change the unit from HZ to bictcp_HZ */ 280 t <<= BICTCP_HZ; 281 do_div(t, HZ); 282 283 if (t < ca->bic_K) /* t - K */ 284 offs = ca->bic_K - t; 285 else 286 offs = t - ca->bic_K; 287 288 /* c/rtt * (t-K)^3 */ 289 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 290 if (t < ca->bic_K) /* below origin*/ 291 bic_target = ca->bic_origin_point - delta; 292 else /* above origin*/ 293 bic_target = ca->bic_origin_point + delta; 294 295 /* cubic function - calc bictcp_cnt*/ 296 if (bic_target > cwnd) { 297 ca->cnt = cwnd / (bic_target - cwnd); 298 } else { 299 ca->cnt = 100 * cwnd; /* very small increment*/ 300 } 301 302 /* 303 * The initial growth of cubic function may be too conservative 304 * when the available bandwidth is still unknown. 305 */ 306 if (ca->last_max_cwnd == 0 && ca->cnt > 20) 307 ca->cnt = 20; /* increase cwnd 5% per RTT */ 308 309 tcp_friendliness: 310 /* TCP Friendly */ 311 if (tcp_friendliness) { 312 u32 scale = beta_scale; 313 314 delta = (cwnd * scale) >> 3; 315 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 316 ca->ack_cnt -= delta; 317 ca->tcp_cwnd++; 318 } 319 320 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */ 321 delta = ca->tcp_cwnd - cwnd; 322 max_cnt = cwnd / delta; 323 if (ca->cnt > max_cnt) 324 ca->cnt = max_cnt; 325 } 326 } 327 328 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per 329 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT. 330 */ 331 ca->cnt = max(ca->cnt, 2U); 332 } 333 334 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) 335 { 336 struct tcp_sock *tp = tcp_sk(sk); 337 struct bictcp *ca = inet_csk_ca(sk); 338 339 if (!tcp_is_cwnd_limited(sk)) 340 return; 341 342 if (tcp_in_slow_start(tp)) { 343 if (hystart && after(ack, ca->end_seq)) 344 bictcp_hystart_reset(sk); 345 acked = tcp_slow_start(tp, acked); 346 if (!acked) 347 return; 348 } 349 bictcp_update(ca, tp->snd_cwnd, acked); 350 tcp_cong_avoid_ai(tp, ca->cnt, acked); 351 } 352 353 static u32 bictcp_recalc_ssthresh(struct sock *sk) 354 { 355 const struct tcp_sock *tp = tcp_sk(sk); 356 struct bictcp *ca = inet_csk_ca(sk); 357 358 ca->epoch_start = 0; /* end of epoch */ 359 360 /* Wmax and fast convergence */ 361 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 362 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 363 / (2 * BICTCP_BETA_SCALE); 364 else 365 ca->last_max_cwnd = tp->snd_cwnd; 366 367 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 368 } 369 370 static void bictcp_state(struct sock *sk, u8 new_state) 371 { 372 if (new_state == TCP_CA_Loss) { 373 bictcp_reset(inet_csk_ca(sk)); 374 bictcp_hystart_reset(sk); 375 } 376 } 377 378 static void hystart_update(struct sock *sk, u32 delay) 379 { 380 struct tcp_sock *tp = tcp_sk(sk); 381 struct bictcp *ca = inet_csk_ca(sk); 382 383 if (ca->found & hystart_detect) 384 return; 385 386 if (hystart_detect & HYSTART_ACK_TRAIN) { 387 u32 now = bictcp_clock(); 388 389 /* first detection parameter - ack-train detection */ 390 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) { 391 ca->last_ack = now; 392 if ((s32)(now - ca->round_start) > ca->delay_min >> 4) { 393 ca->found |= HYSTART_ACK_TRAIN; 394 NET_INC_STATS(sock_net(sk), 395 LINUX_MIB_TCPHYSTARTTRAINDETECT); 396 NET_ADD_STATS(sock_net(sk), 397 LINUX_MIB_TCPHYSTARTTRAINCWND, 398 tp->snd_cwnd); 399 tp->snd_ssthresh = tp->snd_cwnd; 400 } 401 } 402 } 403 404 if (hystart_detect & HYSTART_DELAY) { 405 /* obtain the minimum delay of more than sampling packets */ 406 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { 407 if (ca->curr_rtt == 0 || ca->curr_rtt > delay) 408 ca->curr_rtt = delay; 409 410 ca->sample_cnt++; 411 } else { 412 if (ca->curr_rtt > ca->delay_min + 413 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { 414 ca->found |= HYSTART_DELAY; 415 NET_INC_STATS(sock_net(sk), 416 LINUX_MIB_TCPHYSTARTDELAYDETECT); 417 NET_ADD_STATS(sock_net(sk), 418 LINUX_MIB_TCPHYSTARTDELAYCWND, 419 tp->snd_cwnd); 420 tp->snd_ssthresh = tp->snd_cwnd; 421 } 422 } 423 } 424 } 425 426 /* Track delayed acknowledgment ratio using sliding window 427 * ratio = (15*ratio + sample) / 16 428 */ 429 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) 430 { 431 const struct tcp_sock *tp = tcp_sk(sk); 432 struct bictcp *ca = inet_csk_ca(sk); 433 u32 delay; 434 435 /* Some calls are for duplicates without timetamps */ 436 if (sample->rtt_us < 0) 437 return; 438 439 /* Discard delay samples right after fast recovery */ 440 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) 441 return; 442 443 delay = (sample->rtt_us << 3) / USEC_PER_MSEC; 444 if (delay == 0) 445 delay = 1; 446 447 /* first time call or link delay decreases */ 448 if (ca->delay_min == 0 || ca->delay_min > delay) 449 ca->delay_min = delay; 450 451 /* hystart triggers when cwnd is larger than some threshold */ 452 if (hystart && tcp_in_slow_start(tp) && 453 tp->snd_cwnd >= hystart_low_window) 454 hystart_update(sk, delay); 455 } 456 457 static struct tcp_congestion_ops cubictcp __read_mostly = { 458 .init = bictcp_init, 459 .ssthresh = bictcp_recalc_ssthresh, 460 .cong_avoid = bictcp_cong_avoid, 461 .set_state = bictcp_state, 462 .undo_cwnd = tcp_reno_undo_cwnd, 463 .cwnd_event = bictcp_cwnd_event, 464 .pkts_acked = bictcp_acked, 465 .owner = THIS_MODULE, 466 .name = "cubic", 467 }; 468 469 static int __init cubictcp_register(void) 470 { 471 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 472 473 /* Precompute a bunch of the scaling factors that are used per-packet 474 * based on SRTT of 100ms 475 */ 476 477 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3 478 / (BICTCP_BETA_SCALE - beta); 479 480 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 481 482 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 483 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 484 * the unit of K is bictcp_HZ=2^10, not HZ 485 * 486 * c = bic_scale >> 10 487 * rtt = 100ms 488 * 489 * the following code has been designed and tested for 490 * cwnd < 1 million packets 491 * RTT < 100 seconds 492 * HZ < 1,000,00 (corresponding to 10 nano-second) 493 */ 494 495 /* 1/c * 2^2*bictcp_HZ * srtt */ 496 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 497 498 /* divide by bic_scale and by constant Srtt (100ms) */ 499 do_div(cube_factor, bic_scale * 10); 500 501 return tcp_register_congestion_control(&cubictcp); 502 } 503 504 static void __exit cubictcp_unregister(void) 505 { 506 tcp_unregister_congestion_control(&cubictcp); 507 } 508 509 module_init(cubictcp_register); 510 module_exit(cubictcp_unregister); 511 512 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 513 MODULE_LICENSE("GPL"); 514 MODULE_DESCRIPTION("CUBIC TCP"); 515 MODULE_VERSION("2.3"); 516