1 // SPDX-License-Identifier: GPL-2.0-only 2 /* 3 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3 4 * Home page: 5 * http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC 6 * This is from the implementation of CUBIC TCP in 7 * Sangtae Ha, Injong Rhee and Lisong Xu, 8 * "CUBIC: A New TCP-Friendly High-Speed TCP Variant" 9 * in ACM SIGOPS Operating System Review, July 2008. 10 * Available from: 11 * http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf 12 * 13 * CUBIC integrates a new slow start algorithm, called HyStart. 14 * The details of HyStart are presented in 15 * Sangtae Ha and Injong Rhee, 16 * "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008. 17 * Available from: 18 * http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf 19 * 20 * All testing results are available from: 21 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing 22 * 23 * Unless CUBIC is enabled and congestion window is large 24 * this behaves the same as the original Reno. 25 */ 26 27 #include <linux/mm.h> 28 #include <linux/module.h> 29 #include <linux/math64.h> 30 #include <net/tcp.h> 31 32 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation 33 * max_cwnd = snd_cwnd * beta 34 */ 35 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */ 36 37 /* Two methods of hybrid slow start */ 38 #define HYSTART_ACK_TRAIN 0x1 39 #define HYSTART_DELAY 0x2 40 41 /* Number of delay samples for detecting the increase of delay */ 42 #define HYSTART_MIN_SAMPLES 8 43 #define HYSTART_DELAY_MIN (4U<<3) 44 #define HYSTART_DELAY_MAX (16U<<3) 45 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX) 46 47 static int fast_convergence __read_mostly = 1; 48 static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */ 49 static int initial_ssthresh __read_mostly; 50 static int bic_scale __read_mostly = 41; 51 static int tcp_friendliness __read_mostly = 1; 52 53 static int hystart __read_mostly = 1; 54 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY; 55 static int hystart_low_window __read_mostly = 16; 56 static int hystart_ack_delta __read_mostly = 2; 57 58 static u32 cube_rtt_scale __read_mostly; 59 static u32 beta_scale __read_mostly; 60 static u64 cube_factor __read_mostly; 61 62 /* Note parameters that are used for precomputing scale factors are read-only */ 63 module_param(fast_convergence, int, 0644); 64 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); 65 module_param(beta, int, 0644); 66 MODULE_PARM_DESC(beta, "beta for multiplicative increase"); 67 module_param(initial_ssthresh, int, 0644); 68 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); 69 module_param(bic_scale, int, 0444); 70 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); 71 module_param(tcp_friendliness, int, 0644); 72 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); 73 module_param(hystart, int, 0644); 74 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm"); 75 module_param(hystart_detect, int, 0644); 76 MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms" 77 " 1: packet-train 2: delay 3: both packet-train and delay"); 78 module_param(hystart_low_window, int, 0644); 79 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start"); 80 module_param(hystart_ack_delta, int, 0644); 81 MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)"); 82 83 /* BIC TCP Parameters */ 84 struct bictcp { 85 u32 cnt; /* increase cwnd by 1 after ACKs */ 86 u32 last_max_cwnd; /* last maximum snd_cwnd */ 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 }; 104 105 static 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 } 119 120 static 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 } 128 129 static 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 } 139 140 static void bictcp_init(struct sock *sk) 141 { 142 struct bictcp *ca = inet_csk_ca(sk); 143 144 bictcp_reset(ca); 145 146 if (hystart) 147 bictcp_hystart_reset(sk); 148 149 if (!hystart && initial_ssthresh) 150 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 151 } 152 153 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event) 154 { 155 if (event == CA_EVENT_TX_START) { 156 struct bictcp *ca = inet_csk_ca(sk); 157 u32 now = tcp_jiffies32; 158 s32 delta; 159 160 delta = now - tcp_sk(sk)->lsndtime; 161 162 /* We were application limited (idle) for a while. 163 * Shift epoch_start to keep cwnd growth to cubic curve. 164 */ 165 if (ca->epoch_start && delta > 0) { 166 ca->epoch_start += delta; 167 if (after(ca->epoch_start, now)) 168 ca->epoch_start = now; 169 } 170 return; 171 } 172 } 173 174 /* calculate the cubic root of x using a table lookup followed by one 175 * Newton-Raphson iteration. 176 * Avg err ~= 0.195% 177 */ 178 static u32 cubic_root(u64 a) 179 { 180 u32 x, b, shift; 181 /* 182 * cbrt(x) MSB values for x MSB values in [0..63]. 183 * Precomputed then refined by hand - Willy Tarreau 184 * 185 * For x in [0..63], 186 * v = cbrt(x << 18) - 1 187 * cbrt(x) = (v[x] + 10) >> 6 188 */ 189 static const u8 v[] = { 190 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 191 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 192 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 193 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 194 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 195 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 196 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 197 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 198 }; 199 200 b = fls64(a); 201 if (b < 7) { 202 /* a in [0..63] */ 203 return ((u32)v[(u32)a] + 35) >> 6; 204 } 205 206 b = ((b * 84) >> 8) - 1; 207 shift = (a >> (b * 3)); 208 209 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 210 211 /* 212 * Newton-Raphson iteration 213 * 2 214 * x = ( 2 * x + a / x ) / 3 215 * k+1 k k 216 */ 217 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); 218 x = ((x * 341) >> 10); 219 return x; 220 } 221 222 /* 223 * Compute congestion window to use. 224 */ 225 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked) 226 { 227 u32 delta, bic_target, max_cnt; 228 u64 offs, t; 229 230 ca->ack_cnt += acked; /* count the number of ACKed packets */ 231 232 if (ca->last_cwnd == cwnd && 233 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32) 234 return; 235 236 /* The CUBIC function can update ca->cnt at most once per jiffy. 237 * On all cwnd reduction events, ca->epoch_start is set to 0, 238 * which will force a recalculation of ca->cnt. 239 */ 240 if (ca->epoch_start && tcp_jiffies32 == ca->last_time) 241 goto tcp_friendliness; 242 243 ca->last_cwnd = cwnd; 244 ca->last_time = tcp_jiffies32; 245 246 if (ca->epoch_start == 0) { 247 ca->epoch_start = tcp_jiffies32; /* record beginning */ 248 ca->ack_cnt = acked; /* start counting */ 249 ca->tcp_cwnd = cwnd; /* syn with cubic */ 250 251 if (ca->last_max_cwnd <= cwnd) { 252 ca->bic_K = 0; 253 ca->bic_origin_point = cwnd; 254 } else { 255 /* Compute new K based on 256 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 257 */ 258 ca->bic_K = cubic_root(cube_factor 259 * (ca->last_max_cwnd - cwnd)); 260 ca->bic_origin_point = ca->last_max_cwnd; 261 } 262 } 263 264 /* cubic function - calc*/ 265 /* calculate c * time^3 / rtt, 266 * while considering overflow in calculation of time^3 267 * (so time^3 is done by using 64 bit) 268 * and without the support of division of 64bit numbers 269 * (so all divisions are done by using 32 bit) 270 * also NOTE the unit of those veriables 271 * time = (t - K) / 2^bictcp_HZ 272 * c = bic_scale >> 10 273 * rtt = (srtt >> 3) / HZ 274 * !!! The following code does not have overflow problems, 275 * if the cwnd < 1 million packets !!! 276 */ 277 278 t = (s32)(tcp_jiffies32 - ca->epoch_start); 279 t += msecs_to_jiffies(ca->delay_min >> 3); 280 /* change the unit from HZ to bictcp_HZ */ 281 t <<= BICTCP_HZ; 282 do_div(t, HZ); 283 284 if (t < ca->bic_K) /* t - K */ 285 offs = ca->bic_K - t; 286 else 287 offs = t - ca->bic_K; 288 289 /* c/rtt * (t-K)^3 */ 290 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 291 if (t < ca->bic_K) /* below origin*/ 292 bic_target = ca->bic_origin_point - delta; 293 else /* above origin*/ 294 bic_target = ca->bic_origin_point + delta; 295 296 /* cubic function - calc bictcp_cnt*/ 297 if (bic_target > cwnd) { 298 ca->cnt = cwnd / (bic_target - cwnd); 299 } else { 300 ca->cnt = 100 * cwnd; /* very small increment*/ 301 } 302 303 /* 304 * The initial growth of cubic function may be too conservative 305 * when the available bandwidth is still unknown. 306 */ 307 if (ca->last_max_cwnd == 0 && ca->cnt > 20) 308 ca->cnt = 20; /* increase cwnd 5% per RTT */ 309 310 tcp_friendliness: 311 /* TCP Friendly */ 312 if (tcp_friendliness) { 313 u32 scale = beta_scale; 314 315 delta = (cwnd * scale) >> 3; 316 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 317 ca->ack_cnt -= delta; 318 ca->tcp_cwnd++; 319 } 320 321 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */ 322 delta = ca->tcp_cwnd - cwnd; 323 max_cnt = cwnd / delta; 324 if (ca->cnt > max_cnt) 325 ca->cnt = max_cnt; 326 } 327 } 328 329 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per 330 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT. 331 */ 332 ca->cnt = max(ca->cnt, 2U); 333 } 334 335 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked) 336 { 337 struct tcp_sock *tp = tcp_sk(sk); 338 struct bictcp *ca = inet_csk_ca(sk); 339 340 if (!tcp_is_cwnd_limited(sk)) 341 return; 342 343 if (tcp_in_slow_start(tp)) { 344 if (hystart && after(ack, ca->end_seq)) 345 bictcp_hystart_reset(sk); 346 acked = tcp_slow_start(tp, acked); 347 if (!acked) 348 return; 349 } 350 bictcp_update(ca, tp->snd_cwnd, acked); 351 tcp_cong_avoid_ai(tp, ca->cnt, acked); 352 } 353 354 static u32 bictcp_recalc_ssthresh(struct sock *sk) 355 { 356 const struct tcp_sock *tp = tcp_sk(sk); 357 struct bictcp *ca = inet_csk_ca(sk); 358 359 ca->epoch_start = 0; /* end of epoch */ 360 361 /* Wmax and fast convergence */ 362 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 363 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 364 / (2 * BICTCP_BETA_SCALE); 365 else 366 ca->last_max_cwnd = tp->snd_cwnd; 367 368 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 369 } 370 371 static void bictcp_state(struct sock *sk, u8 new_state) 372 { 373 if (new_state == TCP_CA_Loss) { 374 bictcp_reset(inet_csk_ca(sk)); 375 bictcp_hystart_reset(sk); 376 } 377 } 378 379 static void hystart_update(struct sock *sk, u32 delay) 380 { 381 struct tcp_sock *tp = tcp_sk(sk); 382 struct bictcp *ca = inet_csk_ca(sk); 383 384 if (ca->found & hystart_detect) 385 return; 386 387 if (hystart_detect & HYSTART_ACK_TRAIN) { 388 u32 now = bictcp_clock(); 389 390 /* first detection parameter - ack-train detection */ 391 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) { 392 ca->last_ack = now; 393 if ((s32)(now - ca->round_start) > ca->delay_min >> 4) { 394 ca->found |= HYSTART_ACK_TRAIN; 395 NET_INC_STATS(sock_net(sk), 396 LINUX_MIB_TCPHYSTARTTRAINDETECT); 397 NET_ADD_STATS(sock_net(sk), 398 LINUX_MIB_TCPHYSTARTTRAINCWND, 399 tp->snd_cwnd); 400 tp->snd_ssthresh = tp->snd_cwnd; 401 } 402 } 403 } 404 405 if (hystart_detect & HYSTART_DELAY) { 406 /* obtain the minimum delay of more than sampling packets */ 407 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) { 408 if (ca->curr_rtt == 0 || ca->curr_rtt > delay) 409 ca->curr_rtt = delay; 410 411 ca->sample_cnt++; 412 } else { 413 if (ca->curr_rtt > ca->delay_min + 414 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { 415 ca->found |= HYSTART_DELAY; 416 NET_INC_STATS(sock_net(sk), 417 LINUX_MIB_TCPHYSTARTDELAYDETECT); 418 NET_ADD_STATS(sock_net(sk), 419 LINUX_MIB_TCPHYSTARTDELAYCWND, 420 tp->snd_cwnd); 421 tp->snd_ssthresh = tp->snd_cwnd; 422 } 423 } 424 } 425 } 426 427 /* Track delayed acknowledgment ratio using sliding window 428 * ratio = (15*ratio + sample) / 16 429 */ 430 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) 431 { 432 const struct tcp_sock *tp = tcp_sk(sk); 433 struct bictcp *ca = inet_csk_ca(sk); 434 u32 delay; 435 436 /* Some calls are for duplicates without timetamps */ 437 if (sample->rtt_us < 0) 438 return; 439 440 /* Discard delay samples right after fast recovery */ 441 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ) 442 return; 443 444 delay = (sample->rtt_us << 3) / USEC_PER_MSEC; 445 if (delay == 0) 446 delay = 1; 447 448 /* first time call or link delay decreases */ 449 if (ca->delay_min == 0 || ca->delay_min > delay) 450 ca->delay_min = delay; 451 452 /* hystart triggers when cwnd is larger than some threshold */ 453 if (hystart && tcp_in_slow_start(tp) && 454 tp->snd_cwnd >= hystart_low_window) 455 hystart_update(sk, delay); 456 } 457 458 static struct tcp_congestion_ops cubictcp __read_mostly = { 459 .init = bictcp_init, 460 .ssthresh = bictcp_recalc_ssthresh, 461 .cong_avoid = bictcp_cong_avoid, 462 .set_state = bictcp_state, 463 .undo_cwnd = tcp_reno_undo_cwnd, 464 .cwnd_event = bictcp_cwnd_event, 465 .pkts_acked = bictcp_acked, 466 .owner = THIS_MODULE, 467 .name = "cubic", 468 }; 469 470 static int __init cubictcp_register(void) 471 { 472 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 473 474 /* Precompute a bunch of the scaling factors that are used per-packet 475 * based on SRTT of 100ms 476 */ 477 478 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3 479 / (BICTCP_BETA_SCALE - beta); 480 481 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 482 483 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 484 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 485 * the unit of K is bictcp_HZ=2^10, not HZ 486 * 487 * c = bic_scale >> 10 488 * rtt = 100ms 489 * 490 * the following code has been designed and tested for 491 * cwnd < 1 million packets 492 * RTT < 100 seconds 493 * HZ < 1,000,00 (corresponding to 10 nano-second) 494 */ 495 496 /* 1/c * 2^2*bictcp_HZ * srtt */ 497 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 498 499 /* divide by bic_scale and by constant Srtt (100ms) */ 500 do_div(cube_factor, bic_scale * 10); 501 502 return tcp_register_congestion_control(&cubictcp); 503 } 504 505 static void __exit cubictcp_unregister(void) 506 { 507 tcp_unregister_congestion_control(&cubictcp); 508 } 509 510 module_init(cubictcp_register); 511 module_exit(cubictcp_unregister); 512 513 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 514 MODULE_LICENSE("GPL"); 515 MODULE_DESCRIPTION("CUBIC TCP"); 516 MODULE_VERSION("2.3"); 517