1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
4 * Home page:
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.
10 * Available from:
11 * http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
12 *
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.
17 * Available from:
18 * http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
19 *
20 * All testing results are available from:
21 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
22 *
23 * Unless CUBIC is enabled and congestion window is large
24 * this behaves the same as the original Reno.
25 */
26
27 #include <linux/mm.h>
28 #include <linux/module.h>
29 #include <linux/math64.h>
30 #include <net/tcp.h>
31
32 #define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
33 * max_cwnd = snd_cwnd * beta
34 */
35 #define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */
36
37 /* Two methods of hybrid slow start */
38 #define HYSTART_ACK_TRAIN 0x1
39 #define HYSTART_DELAY 0x2
40
41 /* Number of delay samples for detecting the increase of delay */
42 #define HYSTART_MIN_SAMPLES 8
43 #define HYSTART_DELAY_MIN (4U<<3)
44 #define HYSTART_DELAY_MAX (16U<<3)
45 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
46
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;
52
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 __read_mostly = 2;
57
58 static u32 cube_rtt_scale __read_mostly;
59 static u32 beta_scale __read_mostly;
60 static u64 cube_factor __read_mostly;
61
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, int, 0644);
81 MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
82
83 /* BIC TCP Parameters */
84 struct bictcp {
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 (msec << 3) */
93 u32 epoch_start; /* beginning of an epoch */
94 u32 ack_cnt; /* number of acks */
95 u32 tcp_cwnd; /* estimated tcp cwnd */
96 u16 unused;
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 */
103 };
104
bictcp_reset(struct bictcp * ca)105 static inline void bictcp_reset(struct bictcp *ca)
106 {
107 ca->cnt = 0;
108 ca->last_max_cwnd = 0;
109 ca->last_cwnd = 0;
110 ca->last_time = 0;
111 ca->bic_origin_point = 0;
112 ca->bic_K = 0;
113 ca->delay_min = 0;
114 ca->epoch_start = 0;
115 ca->ack_cnt = 0;
116 ca->tcp_cwnd = 0;
117 ca->found = 0;
118 }
119
bictcp_clock(void)120 static inline u32 bictcp_clock(void)
121 {
122 #if HZ < 1000
123 return ktime_to_ms(ktime_get_real());
124 #else
125 return jiffies_to_msecs(jiffies);
126 #endif
127 }
128
bictcp_hystart_reset(struct sock * sk)129 static inline void bictcp_hystart_reset(struct sock *sk)
130 {
131 struct tcp_sock *tp = tcp_sk(sk);
132 struct bictcp *ca = inet_csk_ca(sk);
133
134 ca->round_start = ca->last_ack = bictcp_clock();
135 ca->end_seq = tp->snd_nxt;
136 ca->curr_rtt = 0;
137 ca->sample_cnt = 0;
138 }
139
bictcp_init(struct sock * sk)140 static void bictcp_init(struct sock *sk)
141 {
142 struct bictcp *ca = inet_csk_ca(sk);
143
144 bictcp_reset(ca);
145
146 if (hystart)
147 bictcp_hystart_reset(sk);
148
149 if (!hystart && initial_ssthresh)
150 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
151 }
152
bictcp_cwnd_event(struct sock * sk,enum tcp_ca_event event)153 static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
154 {
155 if (event == CA_EVENT_TX_START) {
156 struct bictcp *ca = inet_csk_ca(sk);
157 u32 now = tcp_jiffies32;
158 s32 delta;
159
160 delta = now - tcp_sk(sk)->lsndtime;
161
162 /* We were application limited (idle) for a while.
163 * Shift epoch_start to keep cwnd growth to cubic curve.
164 */
165 if (ca->epoch_start && delta > 0) {
166 ca->epoch_start += delta;
167 if (after(ca->epoch_start, now))
168 ca->epoch_start = now;
169 }
170 return;
171 }
172 }
173
174 /* calculate the cubic root of x using a table lookup followed by one
175 * Newton-Raphson iteration.
176 * Avg err ~= 0.195%
177 */
cubic_root(u64 a)178 static u32 cubic_root(u64 a)
179 {
180 u32 x, b, shift;
181 /*
182 * cbrt(x) MSB values for x MSB values in [0..63].
183 * Precomputed then refined by hand - Willy Tarreau
184 *
185 * For x in [0..63],
186 * v = cbrt(x << 18) - 1
187 * cbrt(x) = (v[x] + 10) >> 6
188 */
189 static const u8 v[] = {
190 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118,
191 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156,
192 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179,
193 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199,
194 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215,
195 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229,
196 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242,
197 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254,
198 };
199
200 b = fls64(a);
201 if (b < 7) {
202 /* a in [0..63] */
203 return ((u32)v[(u32)a] + 35) >> 6;
204 }
205
206 b = ((b * 84) >> 8) - 1;
207 shift = (a >> (b * 3));
208
209 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
210
211 /*
212 * Newton-Raphson iteration
213 * 2
214 * x = ( 2 * x + a / x ) / 3
215 * k+1 k k
216 */
217 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
218 x = ((x * 341) >> 10);
219 return x;
220 }
221
222 /*
223 * Compute congestion window to use.
224 */
bictcp_update(struct bictcp * ca,u32 cwnd,u32 acked)225 static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
226 {
227 u32 delta, bic_target, max_cnt;
228 u64 offs, t;
229
230 ca->ack_cnt += acked; /* count the number of ACKed packets */
231
232 if (ca->last_cwnd == cwnd &&
233 (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
234 return;
235
236 /* The CUBIC function can update ca->cnt at most once per jiffy.
237 * On all cwnd reduction events, ca->epoch_start is set to 0,
238 * which will force a recalculation of ca->cnt.
239 */
240 if (ca->epoch_start && tcp_jiffies32 == ca->last_time)
241 goto tcp_friendliness;
242
243 ca->last_cwnd = cwnd;
244 ca->last_time = tcp_jiffies32;
245
246 if (ca->epoch_start == 0) {
247 ca->epoch_start = tcp_jiffies32; /* record beginning */
248 ca->ack_cnt = acked; /* start counting */
249 ca->tcp_cwnd = cwnd; /* syn with cubic */
250
251 if (ca->last_max_cwnd <= cwnd) {
252 ca->bic_K = 0;
253 ca->bic_origin_point = cwnd;
254 } else {
255 /* Compute new K based on
256 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
257 */
258 ca->bic_K = cubic_root(cube_factor
259 * (ca->last_max_cwnd - cwnd));
260 ca->bic_origin_point = ca->last_max_cwnd;
261 }
262 }
263
264 /* cubic function - calc*/
265 /* calculate c * time^3 / rtt,
266 * while considering overflow in calculation of time^3
267 * (so time^3 is done by using 64 bit)
268 * and without the support of division of 64bit numbers
269 * (so all divisions are done by using 32 bit)
270 * also NOTE the unit of those veriables
271 * time = (t - K) / 2^bictcp_HZ
272 * c = bic_scale >> 10
273 * rtt = (srtt >> 3) / HZ
274 * !!! The following code does not have overflow problems,
275 * if the cwnd < 1 million packets !!!
276 */
277
278 t = (s32)(tcp_jiffies32 - ca->epoch_start);
279 t += msecs_to_jiffies(ca->delay_min >> 3);
280 /* change the unit from HZ to bictcp_HZ */
281 t <<= BICTCP_HZ;
282 do_div(t, HZ);
283
284 if (t < ca->bic_K) /* t - K */
285 offs = ca->bic_K - t;
286 else
287 offs = t - ca->bic_K;
288
289 /* c/rtt * (t-K)^3 */
290 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
291 if (t < ca->bic_K) /* below origin*/
292 bic_target = ca->bic_origin_point - delta;
293 else /* above origin*/
294 bic_target = ca->bic_origin_point + delta;
295
296 /* cubic function - calc bictcp_cnt*/
297 if (bic_target > cwnd) {
298 ca->cnt = cwnd / (bic_target - cwnd);
299 } else {
300 ca->cnt = 100 * cwnd; /* very small increment*/
301 }
302
303 /*
304 * The initial growth of cubic function may be too conservative
305 * when the available bandwidth is still unknown.
306 */
307 if (ca->last_max_cwnd == 0 && ca->cnt > 20)
308 ca->cnt = 20; /* increase cwnd 5% per RTT */
309
310 tcp_friendliness:
311 /* TCP Friendly */
312 if (tcp_friendliness) {
313 u32 scale = beta_scale;
314
315 delta = (cwnd * scale) >> 3;
316 while (ca->ack_cnt > delta) { /* update tcp cwnd */
317 ca->ack_cnt -= delta;
318 ca->tcp_cwnd++;
319 }
320
321 if (ca->tcp_cwnd > cwnd) { /* if bic is slower than tcp */
322 delta = ca->tcp_cwnd - cwnd;
323 max_cnt = cwnd / delta;
324 if (ca->cnt > max_cnt)
325 ca->cnt = max_cnt;
326 }
327 }
328
329 /* The maximum rate of cwnd increase CUBIC allows is 1 packet per
330 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
331 */
332 ca->cnt = max(ca->cnt, 2U);
333 }
334
bictcp_cong_avoid(struct sock * sk,u32 ack,u32 acked)335 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
336 {
337 struct tcp_sock *tp = tcp_sk(sk);
338 struct bictcp *ca = inet_csk_ca(sk);
339
340 if (!tcp_is_cwnd_limited(sk))
341 return;
342
343 if (tcp_in_slow_start(tp)) {
344 if (hystart && after(ack, ca->end_seq))
345 bictcp_hystart_reset(sk);
346 acked = tcp_slow_start(tp, acked);
347 if (!acked)
348 return;
349 }
350 bictcp_update(ca, tp->snd_cwnd, acked);
351 tcp_cong_avoid_ai(tp, ca->cnt, acked);
352 }
353
bictcp_recalc_ssthresh(struct sock * sk)354 static u32 bictcp_recalc_ssthresh(struct sock *sk)
355 {
356 const struct tcp_sock *tp = tcp_sk(sk);
357 struct bictcp *ca = inet_csk_ca(sk);
358
359 ca->epoch_start = 0; /* end of epoch */
360
361 /* Wmax and fast convergence */
362 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
363 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
364 / (2 * BICTCP_BETA_SCALE);
365 else
366 ca->last_max_cwnd = tp->snd_cwnd;
367
368 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
369 }
370
bictcp_state(struct sock * sk,u8 new_state)371 static void bictcp_state(struct sock *sk, u8 new_state)
372 {
373 if (new_state == TCP_CA_Loss) {
374 bictcp_reset(inet_csk_ca(sk));
375 bictcp_hystart_reset(sk);
376 }
377 }
378
hystart_update(struct sock * sk,u32 delay)379 static void hystart_update(struct sock *sk, u32 delay)
380 {
381 struct tcp_sock *tp = tcp_sk(sk);
382 struct bictcp *ca = inet_csk_ca(sk);
383
384 if (ca->found & hystart_detect)
385 return;
386
387 if (hystart_detect & HYSTART_ACK_TRAIN) {
388 u32 now = bictcp_clock();
389
390 /* first detection parameter - ack-train detection */
391 if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
392 ca->last_ack = now;
393 if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
394 ca->found |= HYSTART_ACK_TRAIN;
395 NET_INC_STATS(sock_net(sk),
396 LINUX_MIB_TCPHYSTARTTRAINDETECT);
397 NET_ADD_STATS(sock_net(sk),
398 LINUX_MIB_TCPHYSTARTTRAINCWND,
399 tp->snd_cwnd);
400 tp->snd_ssthresh = tp->snd_cwnd;
401 }
402 }
403 }
404
405 if (hystart_detect & HYSTART_DELAY) {
406 /* obtain the minimum delay of more than sampling packets */
407 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
408 if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
409 ca->curr_rtt = delay;
410
411 ca->sample_cnt++;
412 } else {
413 if (ca->curr_rtt > ca->delay_min +
414 HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
415 ca->found |= HYSTART_DELAY;
416 NET_INC_STATS(sock_net(sk),
417 LINUX_MIB_TCPHYSTARTDELAYDETECT);
418 NET_ADD_STATS(sock_net(sk),
419 LINUX_MIB_TCPHYSTARTDELAYCWND,
420 tp->snd_cwnd);
421 tp->snd_ssthresh = tp->snd_cwnd;
422 }
423 }
424 }
425 }
426
427 /* Track delayed acknowledgment ratio using sliding window
428 * ratio = (15*ratio + sample) / 16
429 */
bictcp_acked(struct sock * sk,const struct ack_sample * sample)430 static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
431 {
432 const struct tcp_sock *tp = tcp_sk(sk);
433 struct bictcp *ca = inet_csk_ca(sk);
434 u32 delay;
435
436 /* Some calls are for duplicates without timetamps */
437 if (sample->rtt_us < 0)
438 return;
439
440 /* Discard delay samples right after fast recovery */
441 if (ca->epoch_start && (s32)(tcp_jiffies32 - ca->epoch_start) < HZ)
442 return;
443
444 delay = (sample->rtt_us << 3) / USEC_PER_MSEC;
445 if (delay == 0)
446 delay = 1;
447
448 /* first time call or link delay decreases */
449 if (ca->delay_min == 0 || ca->delay_min > delay)
450 ca->delay_min = delay;
451
452 /* hystart triggers when cwnd is larger than some threshold */
453 if (hystart && tcp_in_slow_start(tp) &&
454 tp->snd_cwnd >= hystart_low_window)
455 hystart_update(sk, delay);
456 }
457
458 static struct tcp_congestion_ops cubictcp __read_mostly = {
459 .init = bictcp_init,
460 .ssthresh = bictcp_recalc_ssthresh,
461 .cong_avoid = bictcp_cong_avoid,
462 .set_state = bictcp_state,
463 .undo_cwnd = tcp_reno_undo_cwnd,
464 .cwnd_event = bictcp_cwnd_event,
465 .pkts_acked = bictcp_acked,
466 .owner = THIS_MODULE,
467 .name = "cubic",
468 };
469
cubictcp_register(void)470 static int __init cubictcp_register(void)
471 {
472 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
473
474 /* Precompute a bunch of the scaling factors that are used per-packet
475 * based on SRTT of 100ms
476 */
477
478 beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
479 / (BICTCP_BETA_SCALE - beta);
480
481 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */
482
483 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
484 * so K = cubic_root( (wmax-cwnd)*rtt/c )
485 * the unit of K is bictcp_HZ=2^10, not HZ
486 *
487 * c = bic_scale >> 10
488 * rtt = 100ms
489 *
490 * the following code has been designed and tested for
491 * cwnd < 1 million packets
492 * RTT < 100 seconds
493 * HZ < 1,000,00 (corresponding to 10 nano-second)
494 */
495
496 /* 1/c * 2^2*bictcp_HZ * srtt */
497 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
498
499 /* divide by bic_scale and by constant Srtt (100ms) */
500 do_div(cube_factor, bic_scale * 10);
501
502 return tcp_register_congestion_control(&cubictcp);
503 }
504
cubictcp_unregister(void)505 static void __exit cubictcp_unregister(void)
506 {
507 tcp_unregister_congestion_control(&cubictcp);
508 }
509
510 module_init(cubictcp_register);
511 module_exit(cubictcp_unregister);
512
513 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
514 MODULE_LICENSE("GPL");
515 MODULE_DESCRIPTION("CUBIC TCP");
516 MODULE_VERSION("2.3");
517