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