1 /* 2 * TCP CUBIC: Binary Increase Congestion control for TCP v2.1 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 __read_mostly = 1; 30 static int max_increment __read_mostly = 16; 31 static int beta __read_mostly = 819; /* = 819/1024 (BICTCP_BETA_SCALE) */ 32 static int initial_ssthresh __read_mostly; 33 static int bic_scale __read_mostly = 41; 34 static int tcp_friendliness __read_mostly = 1; 35 36 static u32 cube_rtt_scale __read_mostly; 37 static u32 beta_scale __read_mostly; 38 static u64 cube_factor __read_mostly; 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 /* BIC TCP Parameters */ 55 struct bictcp { 56 u32 cnt; /* increase cwnd by 1 after ACKs */ 57 u32 last_max_cwnd; /* last maximum snd_cwnd */ 58 u32 loss_cwnd; /* congestion window at last loss */ 59 u32 last_cwnd; /* the last snd_cwnd */ 60 u32 last_time; /* time when updated last_cwnd */ 61 u32 bic_origin_point;/* origin point of bic function */ 62 u32 bic_K; /* time to origin point from the beginning of the current epoch */ 63 u32 delay_min; /* min delay */ 64 u32 epoch_start; /* beginning of an epoch */ 65 u32 ack_cnt; /* number of acks */ 66 u32 tcp_cwnd; /* estimated tcp cwnd */ 67 #define ACK_RATIO_SHIFT 4 68 u32 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */ 69 }; 70 71 static inline void bictcp_reset(struct bictcp *ca) 72 { 73 ca->cnt = 0; 74 ca->last_max_cwnd = 0; 75 ca->loss_cwnd = 0; 76 ca->last_cwnd = 0; 77 ca->last_time = 0; 78 ca->bic_origin_point = 0; 79 ca->bic_K = 0; 80 ca->delay_min = 0; 81 ca->epoch_start = 0; 82 ca->delayed_ack = 2 << ACK_RATIO_SHIFT; 83 ca->ack_cnt = 0; 84 ca->tcp_cwnd = 0; 85 } 86 87 static void bictcp_init(struct sock *sk) 88 { 89 bictcp_reset(inet_csk_ca(sk)); 90 if (initial_ssthresh) 91 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 92 } 93 94 /* calculate the cubic root of x using a table lookup followed by one 95 * Newton-Raphson iteration. 96 * Avg err ~= 0.195% 97 */ 98 static u32 cubic_root(u64 a) 99 { 100 u32 x, b, shift; 101 /* 102 * cbrt(x) MSB values for x MSB values in [0..63]. 103 * Precomputed then refined by hand - Willy Tarreau 104 * 105 * For x in [0..63], 106 * v = cbrt(x << 18) - 1 107 * cbrt(x) = (v[x] + 10) >> 6 108 */ 109 static const u8 v[] = { 110 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 111 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 112 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 113 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 114 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 115 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 116 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 117 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 118 }; 119 120 b = fls64(a); 121 if (b < 7) { 122 /* a in [0..63] */ 123 return ((u32)v[(u32)a] + 35) >> 6; 124 } 125 126 b = ((b * 84) >> 8) - 1; 127 shift = (a >> (b * 3)); 128 129 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 130 131 /* 132 * Newton-Raphson iteration 133 * 2 134 * x = ( 2 * x + a / x ) / 3 135 * k+1 k k 136 */ 137 x = (2 * x + (u32)div64_64(a, (u64)x * (u64)(x - 1))); 138 x = ((x * 341) >> 10); 139 return x; 140 } 141 142 /* 143 * Compute congestion window to use. 144 */ 145 static inline void bictcp_update(struct bictcp *ca, u32 cwnd) 146 { 147 u64 offs; 148 u32 delta, t, bic_target, min_cnt, max_cnt; 149 150 ca->ack_cnt++; /* count the number of ACKs */ 151 152 if (ca->last_cwnd == cwnd && 153 (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32) 154 return; 155 156 ca->last_cwnd = cwnd; 157 ca->last_time = tcp_time_stamp; 158 159 if (ca->epoch_start == 0) { 160 ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */ 161 ca->ack_cnt = 1; /* start counting */ 162 ca->tcp_cwnd = cwnd; /* syn with cubic */ 163 164 if (ca->last_max_cwnd <= cwnd) { 165 ca->bic_K = 0; 166 ca->bic_origin_point = cwnd; 167 } else { 168 /* Compute new K based on 169 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 170 */ 171 ca->bic_K = cubic_root(cube_factor 172 * (ca->last_max_cwnd - cwnd)); 173 ca->bic_origin_point = ca->last_max_cwnd; 174 } 175 } 176 177 /* cubic function - calc*/ 178 /* calculate c * time^3 / rtt, 179 * while considering overflow in calculation of time^3 180 * (so time^3 is done by using 64 bit) 181 * and without the support of division of 64bit numbers 182 * (so all divisions are done by using 32 bit) 183 * also NOTE the unit of those veriables 184 * time = (t - K) / 2^bictcp_HZ 185 * c = bic_scale >> 10 186 * rtt = (srtt >> 3) / HZ 187 * !!! The following code does not have overflow problems, 188 * if the cwnd < 1 million packets !!! 189 */ 190 191 /* change the unit from HZ to bictcp_HZ */ 192 t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start) 193 << BICTCP_HZ) / HZ; 194 195 if (t < ca->bic_K) /* t - K */ 196 offs = ca->bic_K - t; 197 else 198 offs = t - ca->bic_K; 199 200 /* c/rtt * (t-K)^3 */ 201 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 202 if (t < ca->bic_K) /* below origin*/ 203 bic_target = ca->bic_origin_point - delta; 204 else /* above origin*/ 205 bic_target = ca->bic_origin_point + delta; 206 207 /* cubic function - calc bictcp_cnt*/ 208 if (bic_target > cwnd) { 209 ca->cnt = cwnd / (bic_target - cwnd); 210 } else { 211 ca->cnt = 100 * cwnd; /* very small increment*/ 212 } 213 214 if (ca->delay_min > 0) { 215 /* max increment = Smax * rtt / 0.1 */ 216 min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min); 217 218 /* use concave growth when the target is above the origin */ 219 if (ca->cnt < min_cnt && t >= ca->bic_K) 220 ca->cnt = min_cnt; 221 } 222 223 /* slow start and low utilization */ 224 if (ca->loss_cwnd == 0) /* could be aggressive in slow start */ 225 ca->cnt = 50; 226 227 /* TCP Friendly */ 228 if (tcp_friendliness) { 229 u32 scale = beta_scale; 230 delta = (cwnd * scale) >> 3; 231 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 232 ca->ack_cnt -= delta; 233 ca->tcp_cwnd++; 234 } 235 236 if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */ 237 delta = ca->tcp_cwnd - cwnd; 238 max_cnt = cwnd / delta; 239 if (ca->cnt > max_cnt) 240 ca->cnt = max_cnt; 241 } 242 } 243 244 ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack; 245 if (ca->cnt == 0) /* cannot be zero */ 246 ca->cnt = 1; 247 } 248 249 static void bictcp_cong_avoid(struct sock *sk, u32 ack, 250 u32 in_flight, int data_acked) 251 { 252 struct tcp_sock *tp = tcp_sk(sk); 253 struct bictcp *ca = inet_csk_ca(sk); 254 255 if (!tcp_is_cwnd_limited(sk, in_flight)) 256 return; 257 258 if (tp->snd_cwnd <= tp->snd_ssthresh) 259 tcp_slow_start(tp); 260 else { 261 bictcp_update(ca, tp->snd_cwnd); 262 263 /* In dangerous area, increase slowly. 264 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd 265 */ 266 if (tp->snd_cwnd_cnt >= ca->cnt) { 267 if (tp->snd_cwnd < tp->snd_cwnd_clamp) 268 tp->snd_cwnd++; 269 tp->snd_cwnd_cnt = 0; 270 } else 271 tp->snd_cwnd_cnt++; 272 } 273 274 } 275 276 static u32 bictcp_recalc_ssthresh(struct sock *sk) 277 { 278 const struct tcp_sock *tp = tcp_sk(sk); 279 struct bictcp *ca = inet_csk_ca(sk); 280 281 ca->epoch_start = 0; /* end of epoch */ 282 283 /* Wmax and fast convergence */ 284 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 285 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 286 / (2 * BICTCP_BETA_SCALE); 287 else 288 ca->last_max_cwnd = tp->snd_cwnd; 289 290 ca->loss_cwnd = tp->snd_cwnd; 291 292 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 293 } 294 295 static u32 bictcp_undo_cwnd(struct sock *sk) 296 { 297 struct bictcp *ca = inet_csk_ca(sk); 298 299 return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd); 300 } 301 302 static void bictcp_state(struct sock *sk, u8 new_state) 303 { 304 if (new_state == TCP_CA_Loss) 305 bictcp_reset(inet_csk_ca(sk)); 306 } 307 308 /* Track delayed acknowledgment ratio using sliding window 309 * ratio = (15*ratio + sample) / 16 310 */ 311 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us) 312 { 313 const struct inet_connection_sock *icsk = inet_csk(sk); 314 struct bictcp *ca = inet_csk_ca(sk); 315 u32 delay; 316 317 if (icsk->icsk_ca_state == TCP_CA_Open) { 318 cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT; 319 ca->delayed_ack += cnt; 320 } 321 322 /* Some calls are for duplicates without timetamps */ 323 if (rtt_us < 0) 324 return; 325 326 /* Discard delay samples right after fast recovery */ 327 if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ) 328 return; 329 330 delay = usecs_to_jiffies(rtt_us) << 3; 331 if (delay == 0) 332 delay = 1; 333 334 /* first time call or link delay decreases */ 335 if (ca->delay_min == 0 || ca->delay_min > delay) 336 ca->delay_min = delay; 337 } 338 339 static struct tcp_congestion_ops cubictcp = { 340 .init = bictcp_init, 341 .ssthresh = bictcp_recalc_ssthresh, 342 .cong_avoid = bictcp_cong_avoid, 343 .set_state = bictcp_state, 344 .undo_cwnd = bictcp_undo_cwnd, 345 .pkts_acked = bictcp_acked, 346 .owner = THIS_MODULE, 347 .name = "cubic", 348 }; 349 350 static int __init cubictcp_register(void) 351 { 352 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 353 354 /* Precompute a bunch of the scaling factors that are used per-packet 355 * based on SRTT of 100ms 356 */ 357 358 beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta); 359 360 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 361 362 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 363 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 364 * the unit of K is bictcp_HZ=2^10, not HZ 365 * 366 * c = bic_scale >> 10 367 * rtt = 100ms 368 * 369 * the following code has been designed and tested for 370 * cwnd < 1 million packets 371 * RTT < 100 seconds 372 * HZ < 1,000,00 (corresponding to 10 nano-second) 373 */ 374 375 /* 1/c * 2^2*bictcp_HZ * srtt */ 376 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 377 378 /* divide by bic_scale and by constant Srtt (100ms) */ 379 do_div(cube_factor, bic_scale * 10); 380 381 return tcp_register_congestion_control(&cubictcp); 382 } 383 384 static void __exit cubictcp_unregister(void) 385 { 386 tcp_unregister_congestion_control(&cubictcp); 387 } 388 389 module_init(cubictcp_register); 390 module_exit(cubictcp_unregister); 391 392 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 393 MODULE_LICENSE("GPL"); 394 MODULE_DESCRIPTION("CUBIC TCP"); 395 MODULE_VERSION("2.1"); 396