1 /* 2 * TCP CUBIC: Binary Increase Congestion control for TCP v2.2 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 * Injong Rhee, Lisong Xu. 7 * "CUBIC: A New TCP-Friendly High-Speed TCP Variant 8 * in PFLDnet 2005 9 * Available from: 10 * http://netsrv.csc.ncsu.edu/export/cubic-paper.pdf 11 * 12 * Unless CUBIC is enabled and congestion window is large 13 * this behaves the same as the original Reno. 14 */ 15 16 #include <linux/mm.h> 17 #include <linux/module.h> 18 #include <linux/math64.h> 19 #include <net/tcp.h> 20 21 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation 22 * max_cwnd = snd_cwnd * beta 23 */ 24 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */ 25 26 static int fast_convergence __read_mostly = 1; 27 static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */ 28 static int initial_ssthresh __read_mostly; 29 static int bic_scale __read_mostly = 41; 30 static int tcp_friendliness __read_mostly = 1; 31 32 static u32 cube_rtt_scale __read_mostly; 33 static u32 beta_scale __read_mostly; 34 static u64 cube_factor __read_mostly; 35 36 /* Note parameters that are used for precomputing scale factors are read-only */ 37 module_param(fast_convergence, int, 0644); 38 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence"); 39 module_param(beta, int, 0644); 40 MODULE_PARM_DESC(beta, "beta for multiplicative increase"); 41 module_param(initial_ssthresh, int, 0644); 42 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold"); 43 module_param(bic_scale, int, 0444); 44 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)"); 45 module_param(tcp_friendliness, int, 0644); 46 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness"); 47 48 /* BIC TCP Parameters */ 49 struct bictcp { 50 u32 cnt; /* increase cwnd by 1 after ACKs */ 51 u32 last_max_cwnd; /* last maximum snd_cwnd */ 52 u32 loss_cwnd; /* congestion window at last loss */ 53 u32 last_cwnd; /* the last snd_cwnd */ 54 u32 last_time; /* time when updated last_cwnd */ 55 u32 bic_origin_point;/* origin point of bic function */ 56 u32 bic_K; /* time to origin point from the beginning of the current epoch */ 57 u32 delay_min; /* min delay */ 58 u32 epoch_start; /* beginning of an epoch */ 59 u32 ack_cnt; /* number of acks */ 60 u32 tcp_cwnd; /* estimated tcp cwnd */ 61 #define ACK_RATIO_SHIFT 4 62 u32 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */ 63 }; 64 65 static inline void bictcp_reset(struct bictcp *ca) 66 { 67 ca->cnt = 0; 68 ca->last_max_cwnd = 0; 69 ca->loss_cwnd = 0; 70 ca->last_cwnd = 0; 71 ca->last_time = 0; 72 ca->bic_origin_point = 0; 73 ca->bic_K = 0; 74 ca->delay_min = 0; 75 ca->epoch_start = 0; 76 ca->delayed_ack = 2 << ACK_RATIO_SHIFT; 77 ca->ack_cnt = 0; 78 ca->tcp_cwnd = 0; 79 } 80 81 static void bictcp_init(struct sock *sk) 82 { 83 bictcp_reset(inet_csk_ca(sk)); 84 if (initial_ssthresh) 85 tcp_sk(sk)->snd_ssthresh = initial_ssthresh; 86 } 87 88 /* calculate the cubic root of x using a table lookup followed by one 89 * Newton-Raphson iteration. 90 * Avg err ~= 0.195% 91 */ 92 static u32 cubic_root(u64 a) 93 { 94 u32 x, b, shift; 95 /* 96 * cbrt(x) MSB values for x MSB values in [0..63]. 97 * Precomputed then refined by hand - Willy Tarreau 98 * 99 * For x in [0..63], 100 * v = cbrt(x << 18) - 1 101 * cbrt(x) = (v[x] + 10) >> 6 102 */ 103 static const u8 v[] = { 104 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118, 105 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156, 106 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179, 107 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199, 108 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215, 109 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229, 110 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242, 111 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254, 112 }; 113 114 b = fls64(a); 115 if (b < 7) { 116 /* a in [0..63] */ 117 return ((u32)v[(u32)a] + 35) >> 6; 118 } 119 120 b = ((b * 84) >> 8) - 1; 121 shift = (a >> (b * 3)); 122 123 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6; 124 125 /* 126 * Newton-Raphson iteration 127 * 2 128 * x = ( 2 * x + a / x ) / 3 129 * k+1 k k 130 */ 131 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1))); 132 x = ((x * 341) >> 10); 133 return x; 134 } 135 136 /* 137 * Compute congestion window to use. 138 */ 139 static inline void bictcp_update(struct bictcp *ca, u32 cwnd) 140 { 141 u64 offs; 142 u32 delta, t, bic_target, max_cnt; 143 144 ca->ack_cnt++; /* count the number of ACKs */ 145 146 if (ca->last_cwnd == cwnd && 147 (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32) 148 return; 149 150 ca->last_cwnd = cwnd; 151 ca->last_time = tcp_time_stamp; 152 153 if (ca->epoch_start == 0) { 154 ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */ 155 ca->ack_cnt = 1; /* start counting */ 156 ca->tcp_cwnd = cwnd; /* syn with cubic */ 157 158 if (ca->last_max_cwnd <= cwnd) { 159 ca->bic_K = 0; 160 ca->bic_origin_point = cwnd; 161 } else { 162 /* Compute new K based on 163 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ) 164 */ 165 ca->bic_K = cubic_root(cube_factor 166 * (ca->last_max_cwnd - cwnd)); 167 ca->bic_origin_point = ca->last_max_cwnd; 168 } 169 } 170 171 /* cubic function - calc*/ 172 /* calculate c * time^3 / rtt, 173 * while considering overflow in calculation of time^3 174 * (so time^3 is done by using 64 bit) 175 * and without the support of division of 64bit numbers 176 * (so all divisions are done by using 32 bit) 177 * also NOTE the unit of those veriables 178 * time = (t - K) / 2^bictcp_HZ 179 * c = bic_scale >> 10 180 * rtt = (srtt >> 3) / HZ 181 * !!! The following code does not have overflow problems, 182 * if the cwnd < 1 million packets !!! 183 */ 184 185 /* change the unit from HZ to bictcp_HZ */ 186 t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start) 187 << BICTCP_HZ) / HZ; 188 189 if (t < ca->bic_K) /* t - K */ 190 offs = ca->bic_K - t; 191 else 192 offs = t - ca->bic_K; 193 194 /* c/rtt * (t-K)^3 */ 195 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ); 196 if (t < ca->bic_K) /* below origin*/ 197 bic_target = ca->bic_origin_point - delta; 198 else /* above origin*/ 199 bic_target = ca->bic_origin_point + delta; 200 201 /* cubic function - calc bictcp_cnt*/ 202 if (bic_target > cwnd) { 203 ca->cnt = cwnd / (bic_target - cwnd); 204 } else { 205 ca->cnt = 100 * cwnd; /* very small increment*/ 206 } 207 208 /* TCP Friendly */ 209 if (tcp_friendliness) { 210 u32 scale = beta_scale; 211 delta = (cwnd * scale) >> 3; 212 while (ca->ack_cnt > delta) { /* update tcp cwnd */ 213 ca->ack_cnt -= delta; 214 ca->tcp_cwnd++; 215 } 216 217 if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */ 218 delta = ca->tcp_cwnd - cwnd; 219 max_cnt = cwnd / delta; 220 if (ca->cnt > max_cnt) 221 ca->cnt = max_cnt; 222 } 223 } 224 225 ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack; 226 if (ca->cnt == 0) /* cannot be zero */ 227 ca->cnt = 1; 228 } 229 230 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight) 231 { 232 struct tcp_sock *tp = tcp_sk(sk); 233 struct bictcp *ca = inet_csk_ca(sk); 234 235 if (!tcp_is_cwnd_limited(sk, in_flight)) 236 return; 237 238 if (tp->snd_cwnd <= tp->snd_ssthresh) 239 tcp_slow_start(tp); 240 else { 241 bictcp_update(ca, tp->snd_cwnd); 242 243 /* In dangerous area, increase slowly. 244 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd 245 */ 246 if (tp->snd_cwnd_cnt >= ca->cnt) { 247 if (tp->snd_cwnd < tp->snd_cwnd_clamp) 248 tp->snd_cwnd++; 249 tp->snd_cwnd_cnt = 0; 250 } else 251 tp->snd_cwnd_cnt++; 252 } 253 254 } 255 256 static u32 bictcp_recalc_ssthresh(struct sock *sk) 257 { 258 const struct tcp_sock *tp = tcp_sk(sk); 259 struct bictcp *ca = inet_csk_ca(sk); 260 261 ca->epoch_start = 0; /* end of epoch */ 262 263 /* Wmax and fast convergence */ 264 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence) 265 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta)) 266 / (2 * BICTCP_BETA_SCALE); 267 else 268 ca->last_max_cwnd = tp->snd_cwnd; 269 270 ca->loss_cwnd = tp->snd_cwnd; 271 272 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U); 273 } 274 275 static u32 bictcp_undo_cwnd(struct sock *sk) 276 { 277 struct bictcp *ca = inet_csk_ca(sk); 278 279 return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd); 280 } 281 282 static void bictcp_state(struct sock *sk, u8 new_state) 283 { 284 if (new_state == TCP_CA_Loss) 285 bictcp_reset(inet_csk_ca(sk)); 286 } 287 288 /* Track delayed acknowledgment ratio using sliding window 289 * ratio = (15*ratio + sample) / 16 290 */ 291 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us) 292 { 293 const struct inet_connection_sock *icsk = inet_csk(sk); 294 struct bictcp *ca = inet_csk_ca(sk); 295 u32 delay; 296 297 if (icsk->icsk_ca_state == TCP_CA_Open) { 298 cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT; 299 ca->delayed_ack += cnt; 300 } 301 302 /* Some calls are for duplicates without timetamps */ 303 if (rtt_us < 0) 304 return; 305 306 /* Discard delay samples right after fast recovery */ 307 if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ) 308 return; 309 310 delay = usecs_to_jiffies(rtt_us) << 3; 311 if (delay == 0) 312 delay = 1; 313 314 /* first time call or link delay decreases */ 315 if (ca->delay_min == 0 || ca->delay_min > delay) 316 ca->delay_min = delay; 317 } 318 319 static struct tcp_congestion_ops cubictcp = { 320 .init = bictcp_init, 321 .ssthresh = bictcp_recalc_ssthresh, 322 .cong_avoid = bictcp_cong_avoid, 323 .set_state = bictcp_state, 324 .undo_cwnd = bictcp_undo_cwnd, 325 .pkts_acked = bictcp_acked, 326 .owner = THIS_MODULE, 327 .name = "cubic", 328 }; 329 330 static int __init cubictcp_register(void) 331 { 332 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE); 333 334 /* Precompute a bunch of the scaling factors that are used per-packet 335 * based on SRTT of 100ms 336 */ 337 338 beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta); 339 340 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */ 341 342 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3 343 * so K = cubic_root( (wmax-cwnd)*rtt/c ) 344 * the unit of K is bictcp_HZ=2^10, not HZ 345 * 346 * c = bic_scale >> 10 347 * rtt = 100ms 348 * 349 * the following code has been designed and tested for 350 * cwnd < 1 million packets 351 * RTT < 100 seconds 352 * HZ < 1,000,00 (corresponding to 10 nano-second) 353 */ 354 355 /* 1/c * 2^2*bictcp_HZ * srtt */ 356 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */ 357 358 /* divide by bic_scale and by constant Srtt (100ms) */ 359 do_div(cube_factor, bic_scale * 10); 360 361 return tcp_register_congestion_control(&cubictcp); 362 } 363 364 static void __exit cubictcp_unregister(void) 365 { 366 tcp_unregister_congestion_control(&cubictcp); 367 } 368 369 module_init(cubictcp_register); 370 module_exit(cubictcp_unregister); 371 372 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger"); 373 MODULE_LICENSE("GPL"); 374 MODULE_DESCRIPTION("CUBIC TCP"); 375 MODULE_VERSION("2.2"); 376