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 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 56 static u32 cube_rtt_scale __read_mostly; 57 static u32 beta_scale __read_mostly; 58 static u64 cube_factor __read_mostly; 59 60 /* Note parameters that are used for precomputing scale factors are read-only */ 61 module_param(fast_convergence, int, 0644); 62 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); 63 module_param(beta, int, 0644); 64 MODULE_PARM_DESC(beta, "beta for multiplicative increase"); 65 module_param(initial_ssthresh, int, 0644); 66 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); 67 module_param(bic_scale, int, 0444); 68 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); 69 module_param(tcp_friendliness, int, 0644); 70 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); 71 module_param(hystart, int, 0644); 72 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm"); 73 module_param(hystart_detect, int, 0644); 74 MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms" 75 " 1: packet-train 2: delay 3: both packet-train and delay"); 76 module_param(hystart_low_window, int, 0644); 77 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start"); 78 79 /* BIC TCP Parameters */ 80 struct 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 }; 101 102 static 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 } 118 119 static 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 } 129 130 static 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 */ 145 static 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 */ 192 static 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 } 282 283 static 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 } 301 302 static 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 } 320 321 static 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 } 327 328 static 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 } 335 336 static 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 */ 374 static 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 } 407 408 static 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 }; 418 419 static 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 } 452 453 static void __exit cubictcp_unregister(void) 454 { 455 tcp_unregister_congestion_control(&cubictcp); 456 } 457 458 module_init(cubictcp_register); 459 module_exit(cubictcp_unregister); 460 461 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 462 MODULE_LICENSE("GPL"); 463 MODULE_DESCRIPTION("CUBIC TCP"); 464 MODULE_VERSION("2.3"); 465