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/mm.h> 16 #include <linux/module.h> 17 #include <net/tcp.h> 18 #include <asm/div64.h> 19 20 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation 21 * max_cwnd = snd_cwnd * beta 22 */ 23 #define BICTCP_B 4 /* 24 * In binary search, 25 * go to point (max+min)/N 26 */ 27 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */ 28 29 static int fast_convergence = 1; 30 static int max_increment = 16; 31 static int beta = 819; /* = 819/1024 (BICTCP_BETA_SCALE) */ 32 static int initial_ssthresh = 100; 33 static int bic_scale = 41; 34 static int tcp_friendliness = 1; 35 36 static u32 cube_rtt_scale; 37 static u32 beta_scale; 38 static u64 cube_factor; 39 40 /* Note parameters that are used for precomputing scale factors are read-only */ 41 module_param(fast_convergence, int, 0644); 42 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); 43 module_param(max_increment, int, 0644); 44 MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search"); 45 module_param(beta, int, 0444); 46 MODULE_PARM_DESC(beta, "beta for multiplicative increase"); 47 module_param(initial_ssthresh, int, 0644); 48 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); 49 module_param(bic_scale, int, 0444); 50 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); 51 module_param(tcp_friendliness, int, 0644); 52 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); 53 54 #include <asm/div64.h> 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 /* 64bit divisor, dividend and result. dynamic precision */ 97 static inline u_int64_t div64_64(u_int64_t dividend, u_int64_t divisor) 98 { 99 u_int32_t d = divisor; 100 101 if (divisor > 0xffffffffULL) { 102 unsigned int shift = fls(divisor >> 32); 103 104 d = divisor >> shift; 105 dividend >>= shift; 106 } 107 108 /* avoid 64 bit division if possible */ 109 if (dividend >> 32) 110 do_div(dividend, d); 111 else 112 dividend = (uint32_t) dividend / d; 113 114 return dividend; 115 } 116 117 /* 118 * calculate the cubic root of x using Newton-Raphson 119 */ 120 static u32 cubic_root(u64 a) 121 { 122 u32 x, x1; 123 124 /* Initial estimate is based on: 125 * cbrt(x) = exp(log(x) / 3) 126 */ 127 x = 1u << (fls64(a)/3); 128 129 /* 130 * Iteration based on: 131 * 2 132 * x = ( 2 * x + a / x ) / 3 133 * k+1 k k 134 */ 135 do { 136 x1 = x; 137 x = (2 * x + (uint32_t) div64_64(a, x*x)) / 3; 138 } while (abs(x1 - x) > 1); 139 140 return x; 141 } 142 143 /* 144 * Compute congestion window to use. 145 */ 146 static inline void bictcp_update(struct bictcp *ca, u32 cwnd) 147 { 148 u64 offs; 149 u32 delta, t, bic_target, min_cnt, max_cnt; 150 151 ca->ack_cnt++; /* count the number of ACKs */ 152 153 if (ca->last_cwnd == cwnd && 154 (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32) 155 return; 156 157 ca->last_cwnd = cwnd; 158 ca->last_time = tcp_time_stamp; 159 160 if (ca->epoch_start == 0) { 161 ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */ 162 ca->ack_cnt = 1; /* start counting */ 163 ca->tcp_cwnd = cwnd; /* syn with cubic */ 164 165 if (ca->last_max_cwnd <= cwnd) { 166 ca->bic_K = 0; 167 ca->bic_origin_point = cwnd; 168 } else { 169 /* Compute new K based on 170 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 171 */ 172 ca->bic_K = cubic_root(cube_factor 173 * (ca->last_max_cwnd - cwnd)); 174 ca->bic_origin_point = ca->last_max_cwnd; 175 } 176 } 177 178 /* cubic function - calc*/ 179 /* calculate c * time^3 / rtt, 180 * while considering overflow in calculation of time^3 181 * (so time^3 is done by using 64 bit) 182 * and without the support of division of 64bit numbers 183 * (so all divisions are done by using 32 bit) 184 * also NOTE the unit of those veriables 185 * time = (t - K) / 2^bictcp_HZ 186 * c = bic_scale >> 10 187 * rtt = (srtt >> 3) / HZ 188 * !!! The following code does not have overflow problems, 189 * if the cwnd < 1 million packets !!! 190 */ 191 192 /* change the unit from HZ to bictcp_HZ */ 193 t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start) 194 << BICTCP_HZ) / HZ; 195 196 if (t < ca->bic_K) /* t - K */ 197 offs = ca->bic_K - t; 198 else 199 offs = t - ca->bic_K; 200 201 /* c/rtt * (t-K)^3 */ 202 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 203 if (t < ca->bic_K) /* below origin*/ 204 bic_target = ca->bic_origin_point - delta; 205 else /* above origin*/ 206 bic_target = ca->bic_origin_point + delta; 207 208 /* cubic function - calc bictcp_cnt*/ 209 if (bic_target > cwnd) { 210 ca->cnt = cwnd / (bic_target - cwnd); 211 } else { 212 ca->cnt = 100 * cwnd; /* very small increment*/ 213 } 214 215 if (ca->delay_min > 0) { 216 /* max increment = Smax * rtt / 0.1 */ 217 min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min); 218 if (ca->cnt < min_cnt) 219 ca->cnt = min_cnt; 220 } 221 222 /* slow start and low utilization */ 223 if (ca->loss_cwnd == 0) /* could be aggressive in slow start */ 224 ca->cnt = 50; 225 226 /* TCP Friendly */ 227 if (tcp_friendliness) { 228 u32 scale = beta_scale; 229 delta = (cwnd * scale) >> 3; 230 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 231 ca->ack_cnt -= delta; 232 ca->tcp_cwnd++; 233 } 234 235 if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */ 236 delta = ca->tcp_cwnd - cwnd; 237 max_cnt = cwnd / delta; 238 if (ca->cnt > max_cnt) 239 ca->cnt = max_cnt; 240 } 241 } 242 243 ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack; 244 if (ca->cnt == 0) /* cannot be zero */ 245 ca->cnt = 1; 246 } 247 248 249 /* Keep track of minimum rtt */ 250 static inline void measure_delay(struct sock *sk) 251 { 252 const struct tcp_sock *tp = tcp_sk(sk); 253 struct bictcp *ca = inet_csk_ca(sk); 254 u32 delay; 255 256 /* No time stamp */ 257 if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) || 258 /* Discard delay samples right after fast recovery */ 259 (s32)(tcp_time_stamp - ca->epoch_start) < HZ) 260 return; 261 262 delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr; 263 if (delay == 0) 264 delay = 1; 265 266 /* first time call or link delay decreases */ 267 if (ca->delay_min == 0 || ca->delay_min > delay) 268 ca->delay_min = delay; 269 } 270 271 static void bictcp_cong_avoid(struct sock *sk, u32 ack, 272 u32 seq_rtt, u32 in_flight, int data_acked) 273 { 274 struct tcp_sock *tp = tcp_sk(sk); 275 struct bictcp *ca = inet_csk_ca(sk); 276 277 if (data_acked) 278 measure_delay(sk); 279 280 if (!tcp_is_cwnd_limited(sk, in_flight)) 281 return; 282 283 if (tp->snd_cwnd <= tp->snd_ssthresh) 284 tcp_slow_start(tp); 285 else { 286 bictcp_update(ca, tp->snd_cwnd); 287 288 /* In dangerous area, increase slowly. 289 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd 290 */ 291 if (tp->snd_cwnd_cnt >= ca->cnt) { 292 if (tp->snd_cwnd < tp->snd_cwnd_clamp) 293 tp->snd_cwnd++; 294 tp->snd_cwnd_cnt = 0; 295 } else 296 tp->snd_cwnd_cnt++; 297 } 298 299 } 300 301 static u32 bictcp_recalc_ssthresh(struct sock *sk) 302 { 303 const struct tcp_sock *tp = tcp_sk(sk); 304 struct bictcp *ca = inet_csk_ca(sk); 305 306 ca->epoch_start = 0; /* end of epoch */ 307 308 /* Wmax and fast convergence */ 309 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 310 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 311 / (2 * BICTCP_BETA_SCALE); 312 else 313 ca->last_max_cwnd = tp->snd_cwnd; 314 315 ca->loss_cwnd = tp->snd_cwnd; 316 317 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 318 } 319 320 static u32 bictcp_undo_cwnd(struct sock *sk) 321 { 322 struct bictcp *ca = inet_csk_ca(sk); 323 324 return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd); 325 } 326 327 static void bictcp_state(struct sock *sk, u8 new_state) 328 { 329 if (new_state == TCP_CA_Loss) 330 bictcp_reset(inet_csk_ca(sk)); 331 } 332 333 /* Track delayed acknowledgment ratio using sliding window 334 * ratio = (15*ratio + sample) / 16 335 */ 336 static void bictcp_acked(struct sock *sk, u32 cnt) 337 { 338 const struct inet_connection_sock *icsk = inet_csk(sk); 339 340 if (cnt > 0 && icsk->icsk_ca_state == TCP_CA_Open) { 341 struct bictcp *ca = inet_csk_ca(sk); 342 cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT; 343 ca->delayed_ack += cnt; 344 } 345 } 346 347 348 static struct tcp_congestion_ops cubictcp = { 349 .init = bictcp_init, 350 .ssthresh = bictcp_recalc_ssthresh, 351 .cong_avoid = bictcp_cong_avoid, 352 .set_state = bictcp_state, 353 .undo_cwnd = bictcp_undo_cwnd, 354 .pkts_acked = bictcp_acked, 355 .owner = THIS_MODULE, 356 .name = "cubic", 357 }; 358 359 static int __init cubictcp_register(void) 360 { 361 BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 362 363 /* Precompute a bunch of the scaling factors that are used per-packet 364 * based on SRTT of 100ms 365 */ 366 367 beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta); 368 369 cube_rtt_scale = (bic_scale << 3) / 10; /* 1024*c/rtt */ 370 371 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 372 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 373 * the unit of K is bictcp_HZ=2^10, not HZ 374 * 375 * c = bic_scale >> 10 376 * rtt = 100ms 377 * 378 * the following code has been designed and tested for 379 * cwnd < 1 million packets 380 * RTT < 100 seconds 381 * HZ < 1,000,00 (corresponding to 10 nano-second) 382 */ 383 384 /* 1/c * 2^2*bictcp_HZ * srtt */ 385 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 386 387 /* divide by bic_scale and by constant Srtt (100ms) */ 388 do_div(cube_factor, bic_scale * 10); 389 390 return tcp_register_congestion_control(&cubictcp); 391 } 392 393 static void __exit cubictcp_unregister(void) 394 { 395 tcp_unregister_congestion_control(&cubictcp); 396 } 397 398 module_init(cubictcp_register); 399 module_exit(cubictcp_unregister); 400 401 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 402 MODULE_LICENSE("GPL"); 403 MODULE_DESCRIPTION("CUBIC TCP"); 404 MODULE_VERSION("2.0"); 405