1 // SPDX-License-Identifier: GPL-2.0-only
3 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
5 * http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
6 * This is from the implementation of CUBIC TCP in
7 * Sangtae Ha, Injong Rhee and Lisong Xu,
8 * "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
9 * in ACM SIGOPS Operating System Review, July 2008.
11 * http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
13 * CUBIC integrates a new slow start algorithm, called HyStart.
14 * The details of HyStart are presented in
15 * Sangtae Ha and Injong Rhee,
16 * "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
18 * http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
20 * All testing results are available from:
21 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
23 * Unless CUBIC is enabled and congestion window is large
24 * this behaves the same as the original Reno.
28 #include <linux/module.h>
29 #include <linux/math64.h>
32 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
33 * max_cwnd = snd_cwnd * beta
35 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */
37 /* Two methods of hybrid slow start */
38 #define HYSTART_ACK_TRAIN 0x1
39 #define HYSTART_DELAY 0x2
41 /* Number of delay samples for detecting the increase of delay */
42 #define HYSTART_MIN_SAMPLES 8
43 #define HYSTART_DELAY_MIN (4000U) /* 4 ms */
44 #define HYSTART_DELAY_MAX (16000U) /* 16 ms */
45 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
47 static int fast_convergence __read_mostly = 1;
48 static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */
49 static int initial_ssthresh __read_mostly;
50 static int bic_scale __read_mostly = 41;
51 static int tcp_friendliness __read_mostly = 1;
53 static int hystart __read_mostly = 1;
54 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
55 static int hystart_low_window __read_mostly = 16;
56 static int hystart_ack_delta_us __read_mostly = 2000;
58 static u32 cube_rtt_scale __read_mostly;
59 static u32 beta_scale __read_mostly;
60 static u64 cube_factor __read_mostly;
62 /* Note parameters that are used for precomputing scale factors are read-only */
63 module_param(fast_convergence, int, 0644);
64 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
65 module_param(beta, int, 0644);
66 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
67 module_param(initial_ssthresh, int, 0644);
68 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
69 module_param(bic_scale, int, 0444);
70 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
71 module_param(tcp_friendliness, int, 0644);
72 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
73 module_param(hystart, int, 0644);
74 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
75 module_param(hystart_detect, int, 0644);
76 MODULE_PARM_DESC(hystart_detect, "hybrid slow start detection mechanisms"
77 " 1: packet-train 2: delay 3: both packet-train and delay");
78 module_param(hystart_low_window, int, 0644);
79 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
80 module_param(hystart_ack_delta_us, int, 0644);
81 MODULE_PARM_DESC(hystart_ack_delta_us, "spacing between ack's indicating train (usecs)");
83 /* BIC TCP Parameters */
85 u32 cnt; /* increase cwnd by 1 after ACKs */
86 u32 last_max_cwnd; /* last maximum snd_cwnd */
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
91 from the beginning of the current epoch */
92 u32 delay_min; /* min delay (usec) */
93 u32 epoch_start; /* beginning of an epoch */
94 u32 ack_cnt; /* number of acks */
95 u32 tcp_cwnd; /* estimated tcp cwnd */
97 u8 sample_cnt; /* number of samples to decide curr_rtt */
98 u8 found; /* the exit point is found? */
99 u32 round_start; /* beginning of each round */
100 u32 end_seq; /* end_seq of the round */
101 u32 last_ack; /* last time when the ACK spacing is close */
102 u32 curr_rtt; /* the minimum rtt of current round */
105 static inline void bictcp_reset(struct bictcp *ca)
108 ca->last_max_cwnd = 0;
111 ca->bic_origin_point = 0;
120 static inline u32 bictcp_clock_us(const struct sock *sk)
122 return tcp_sk(sk)->tcp_mstamp;
125 static inline void bictcp_hystart_reset(struct sock *sk)
127 struct tcp_sock *tp = tcp_sk(sk);
128 struct bictcp *ca = inet_csk_ca(sk);
130 ca->round_start = ca->last_ack = bictcp_clock_us(sk);
131 ca->end_seq = tp->snd_nxt;
136 static void bictcp_init(struct sock *sk)
138 struct bictcp *ca = inet_csk_ca(sk);
143 bictcp_hystart_reset(sk);
145 if (!hystart && initial_ssthresh)
146 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
149 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
151 if (event == CA_EVENT_TX_START) {
152 struct bictcp *ca = inet_csk_ca(sk);
153 u32 now = tcp_jiffies32;
156 delta = now - tcp_sk(sk)->lsndtime;
158 /* We were application limited (idle) for a while.
159 * Shift epoch_start to keep cwnd growth to cubic curve.
161 if (ca->epoch_start && delta > 0) {
162 ca->epoch_start += delta;
163 if (after(ca->epoch_start, now))
164 ca->epoch_start = now;
170 /* calculate the cubic root of x using a table lookup followed by one
171 * Newton-Raphson iteration.
174 static u32 cubic_root(u64 a)
178 * cbrt(x) MSB values for x MSB values in [0..63].
179 * Precomputed then refined by hand - Willy Tarreau
182 * v = cbrt(x << 18) - 1
183 * cbrt(x) = (v[x] + 10) >> 6
185 static const u8 v[] = {
186 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118,
187 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156,
188 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179,
189 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199,
190 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215,
191 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229,
192 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242,
193 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254,
199 return ((u32)v[(u32)a] + 35) >> 6;
202 b = ((b * 84) >> 8) - 1;
203 shift = (a >> (b * 3));
205 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
208 * Newton-Raphson iteration
210 * x = ( 2 * x + a / x ) / 3
213 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
214 x = ((x * 341) >> 10);
219 * Compute congestion window to use.
221 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
223 u32 delta, bic_target, max_cnt;
226 ca->ack_cnt += acked; /* count the number of ACKed packets */
228 if (ca->last_cwnd == cwnd &&
229 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
232 /* The CUBIC function can update ca->cnt at most once per jiffy.
233 * On all cwnd reduction events, ca->epoch_start is set to 0,
234 * which will force a recalculation of ca->cnt.
236 if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
237 goto tcp_friendliness;
239 ca->last_cwnd = cwnd;
240 ca->last_time = tcp_jiffies32;
242 if (ca->epoch_start == 0) {
243 ca->epoch_start = tcp_jiffies32; /* record beginning */
244 ca->ack_cnt = acked; /* start counting */
245 ca->tcp_cwnd = cwnd; /* syn with cubic */
247 if (ca->last_max_cwnd <= cwnd) {
249 ca->bic_origin_point = cwnd;
251 /* Compute new K based on
252 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
254 ca->bic_K = cubic_root(cube_factor
255 * (ca->last_max_cwnd - cwnd));
256 ca->bic_origin_point = ca->last_max_cwnd;
260 /* cubic function - calc*/
261 /* calculate c * time^3 / rtt,
262 * while considering overflow in calculation of time^3
263 * (so time^3 is done by using 64 bit)
264 * and without the support of division of 64bit numbers
265 * (so all divisions are done by using 32 bit)
266 * also NOTE the unit of those veriables
267 * time = (t - K) / 2^bictcp_HZ
268 * c = bic_scale >> 10
269 * rtt = (srtt >> 3) / HZ
270 * !!! The following code does not have overflow problems,
271 * if the cwnd < 1 million packets !!!
274 t = (s32)(tcp_jiffies32 - ca->epoch_start);
275 t += usecs_to_jiffies(ca->delay_min);
276 /* change the unit from HZ to bictcp_HZ */
280 if (t < ca->bic_K) /* t - K */
281 offs = ca->bic_K - t;
283 offs = t - ca->bic_K;
285 /* c/rtt * (t-K)^3 */
286 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
287 if (t < ca->bic_K) /* below origin*/
288 bic_target = ca->bic_origin_point - delta;
289 else /* above origin*/
290 bic_target = ca->bic_origin_point + delta;
292 /* cubic function - calc bictcp_cnt*/
293 if (bic_target > cwnd) {
294 ca->cnt = cwnd / (bic_target - cwnd);
296 ca->cnt = 100 * cwnd; /* very small increment*/
300 * The initial growth of cubic function may be too conservative
301 * when the available bandwidth is still unknown.
303 if (ca->last_max_cwnd == 0 && ca->cnt > 20)
304 ca->cnt = 20; /* increase cwnd 5% per RTT */
308 if (tcp_friendliness) {
309 u32 scale = beta_scale;
311 delta = (cwnd * scale) >> 3;
312 while (ca->ack_cnt > delta) { /* update tcp cwnd */
313 ca->ack_cnt -= delta;
317 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */
318 delta = ca->tcp_cwnd - cwnd;
319 max_cnt = cwnd / delta;
320 if (ca->cnt > max_cnt)
325 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
326 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
328 ca->cnt = max(ca->cnt, 2U);
331 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
333 struct tcp_sock *tp = tcp_sk(sk);
334 struct bictcp *ca = inet_csk_ca(sk);
336 if (!tcp_is_cwnd_limited(sk))
339 if (tcp_in_slow_start(tp)) {
340 if (hystart && after(ack, ca->end_seq))
341 bictcp_hystart_reset(sk);
342 acked = tcp_slow_start(tp, acked);
346 bictcp_update(ca, tp->snd_cwnd, acked);
347 tcp_cong_avoid_ai(tp, ca->cnt, acked);
350 static u32 bictcp_recalc_ssthresh(struct sock *sk)
352 const struct tcp_sock *tp = tcp_sk(sk);
353 struct bictcp *ca = inet_csk_ca(sk);
355 ca->epoch_start = 0; /* end of epoch */
357 /* Wmax and fast convergence */
358 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
359 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
360 / (2 * BICTCP_BETA_SCALE);
362 ca->last_max_cwnd = tp->snd_cwnd;
364 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
367 static void bictcp_state(struct sock *sk, u8 new_state)
369 if (new_state == TCP_CA_Loss) {
370 bictcp_reset(inet_csk_ca(sk));
371 bictcp_hystart_reset(sk);
375 /* Account for TSO/GRO delays.
376 * Otherwise short RTT flows could get too small ssthresh, since during
377 * slow start we begin with small TSO packets and ca->delay_min would
378 * not account for long aggregation delay when TSO packets get bigger.
379 * Ideally even with a very small RTT we would like to have at least one
380 * TSO packet being sent and received by GRO, and another one in qdisc layer.
381 * We apply another 100% factor because @rate is doubled at this point.
382 * We cap the cushion to 1ms.
384 static u32 hystart_ack_delay(struct sock *sk)
388 rate = READ_ONCE(sk->sk_pacing_rate);
391 return min_t(u64, USEC_PER_MSEC,
392 div64_ul((u64)GSO_MAX_SIZE * 4 * USEC_PER_SEC, rate));
395 static void hystart_update(struct sock *sk, u32 delay)
397 struct tcp_sock *tp = tcp_sk(sk);
398 struct bictcp *ca = inet_csk_ca(sk);
401 if (hystart_detect & HYSTART_ACK_TRAIN) {
402 u32 now = bictcp_clock_us(sk);
404 /* first detection parameter - ack-train detection */
405 if ((s32)(now - ca->last_ack) <= hystart_ack_delta_us) {
408 threshold = ca->delay_min + hystart_ack_delay(sk);
410 /* Hystart ack train triggers if we get ack past
412 * Pacing might have delayed packets up to RTT/2
415 if (sk->sk_pacing_status == SK_PACING_NONE)
418 if ((s32)(now - ca->round_start) > threshold) {
420 pr_debug("hystart_ack_train (%u > %u) delay_min %u (+ ack_delay %u) cwnd %u\n",
421 now - ca->round_start, threshold,
422 ca->delay_min, hystart_ack_delay(sk), tp->snd_cwnd);
423 NET_INC_STATS(sock_net(sk),
424 LINUX_MIB_TCPHYSTARTTRAINDETECT);
425 NET_ADD_STATS(sock_net(sk),
426 LINUX_MIB_TCPHYSTARTTRAINCWND,
428 tp->snd_ssthresh = tp->snd_cwnd;
433 if (hystart_detect & HYSTART_DELAY) {
434 /* obtain the minimum delay of more than sampling packets */
435 if (ca->curr_rtt > delay)
436 ca->curr_rtt = delay;
437 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
440 if (ca->curr_rtt > ca->delay_min +
441 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
443 NET_INC_STATS(sock_net(sk),
444 LINUX_MIB_TCPHYSTARTDELAYDETECT);
445 NET_ADD_STATS(sock_net(sk),
446 LINUX_MIB_TCPHYSTARTDELAYCWND,
448 tp->snd_ssthresh = tp->snd_cwnd;
454 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
456 const struct tcp_sock *tp = tcp_sk(sk);
457 struct bictcp *ca = inet_csk_ca(sk);
460 /* Some calls are for duplicates without timetamps */
461 if (sample->rtt_us < 0)
464 /* Discard delay samples right after fast recovery */
465 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
468 delay = sample->rtt_us;
472 /* first time call or link delay decreases */
473 if (ca->delay_min == 0 || ca->delay_min > delay)
474 ca->delay_min = delay;
476 /* hystart triggers when cwnd is larger than some threshold */
477 if (!ca->found && tcp_in_slow_start(tp) && hystart &&
478 tp->snd_cwnd >= hystart_low_window)
479 hystart_update(sk, delay);
482 static struct tcp_congestion_ops cubictcp __read_mostly = {
484 .ssthresh = bictcp_recalc_ssthresh,
485 .cong_avoid = bictcp_cong_avoid,
486 .set_state = bictcp_state,
487 .undo_cwnd = tcp_reno_undo_cwnd,
488 .cwnd_event = bictcp_cwnd_event,
489 .pkts_acked = bictcp_acked,
490 .owner = THIS_MODULE,
494 static int __init cubictcp_register(void)
496 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
498 /* Precompute a bunch of the scaling factors that are used per-packet
499 * based on SRTT of 100ms
502 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
503 / (BICTCP_BETA_SCALE - beta);
505 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */
507 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
508 * so K = cubic_root( (wmax-cwnd)*rtt/c )
509 * the unit of K is bictcp_HZ=2^10, not HZ
511 * c = bic_scale >> 10
514 * the following code has been designed and tested for
515 * cwnd < 1 million packets
517 * HZ < 1,000,00 (corresponding to 10 nano-second)
520 /* 1/c * 2^2*bictcp_HZ * srtt */
521 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
523 /* divide by bic_scale and by constant Srtt (100ms) */
524 do_div(cube_factor, bic_scale * 10);
526 return tcp_register_congestion_control(&cubictcp);
529 static void __exit cubictcp_unregister(void)
531 tcp_unregister_congestion_control(&cubictcp);
534 module_init(cubictcp_register);
535 module_exit(cubictcp_unregister);
537 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
538 MODULE_LICENSE("GPL");
539 MODULE_DESCRIPTION("CUBIC TCP");
540 MODULE_VERSION("2.3");