1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2008-2010 Lawrence Stewart <lstewart@freebsd.org>
5 * Copyright (c) 2010 The FreeBSD Foundation
6 * All rights reserved.
7 *
8 * This software was developed by Lawrence Stewart while studying at the Centre
9 * for Advanced Internet Architectures, Swinburne University of Technology, made
10 * possible in part by a grant from the Cisco University Research Program Fund
11 * at Community Foundation Silicon Valley.
12 *
13 * Portions of this software were developed at the Centre for Advanced
14 * Internet Architectures, Swinburne University of Technology, Melbourne,
15 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
16 *
17 * Redistribution and use in source and binary forms, with or without
18 * modification, are permitted provided that the following conditions
19 * are met:
20 * 1. Redistributions of source code must retain the above copyright
21 * notice, this list of conditions and the following disclaimer.
22 * 2. Redistributions in binary form must reproduce the above copyright
23 * notice, this list of conditions and the following disclaimer in the
24 * documentation and/or other materials provided with the distribution.
25 *
26 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
27 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
28 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
29 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
30 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
31 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
32 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
33 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
34 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
35 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
36 * SUCH DAMAGE.
37 */
38
39 /*
40 * An implementation of the CUBIC congestion control algorithm for FreeBSD,
41 * based on the Internet RFC9438 by Xu, Ha, Rhee, Goel, and Eggert.
42 * Originally released as part of the NewTCP research project at Swinburne
43 * University of Technology's Centre for Advanced Internet Architectures,
44 * Melbourne, Australia, which was made possible in part by a grant from the
45 * Cisco University Research Program Fund at Community Foundation Silicon
46 * Valley. More details are available at:
47 * http://caia.swin.edu.au/urp/newtcp/
48 */
49
50 #include <sys/param.h>
51 #include <sys/kernel.h>
52 #include <sys/limits.h>
53 #include <sys/malloc.h>
54 #include <sys/module.h>
55 #include <sys/socket.h>
56 #include <sys/socketvar.h>
57 #include <sys/sysctl.h>
58 #include <sys/systm.h>
59
60 #include <net/vnet.h>
61
62 #include <net/route.h>
63 #include <net/route/nhop.h>
64
65 #include <netinet/in_pcb.h>
66 #include <netinet/tcp.h>
67 #include <netinet/tcp_seq.h>
68 #include <netinet/tcp_timer.h>
69 #include <netinet/tcp_var.h>
70 #include <netinet/tcp_log_buf.h>
71 #include <netinet/tcp_hpts.h>
72 #include <netinet/cc/cc.h>
73 #include <netinet/cc/cc_cubic.h>
74 #include <netinet/cc/cc_module.h>
75
76 static void cubic_ack_received(struct cc_var *ccv, ccsignal_t type);
77 static void cubic_cb_destroy(struct cc_var *ccv);
78 static int cubic_cb_init(struct cc_var *ccv, void *ptr);
79 static void cubic_cong_signal(struct cc_var *ccv, ccsignal_t type);
80 static void cubic_conn_init(struct cc_var *ccv);
81 static int cubic_mod_init(void);
82 static void cubic_post_recovery(struct cc_var *ccv);
83 static void cubic_record_rtt(struct cc_var *ccv);
84 static uint32_t cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg);
85 static void cubic_after_idle(struct cc_var *ccv);
86 static size_t cubic_data_sz(void);
87 static void cubic_newround(struct cc_var *ccv, uint32_t round_cnt);
88 static void cubic_rttsample(struct cc_var *ccv, uint32_t usec_rtt,
89 uint32_t rxtcnt, uint32_t fas);
90
91 struct cc_algo cubic_cc_algo = {
92 .name = "cubic",
93 .ack_received = cubic_ack_received,
94 .cb_destroy = cubic_cb_destroy,
95 .cb_init = cubic_cb_init,
96 .cong_signal = cubic_cong_signal,
97 .conn_init = cubic_conn_init,
98 .mod_init = cubic_mod_init,
99 .post_recovery = cubic_post_recovery,
100 .after_idle = cubic_after_idle,
101 .cc_data_sz = cubic_data_sz,
102 .rttsample = cubic_rttsample,
103 .newround = cubic_newround
104 };
105
106 static void
cubic_log_hystart_event(struct cc_var * ccv,struct cubic * cubicd,uint8_t mod,uint32_t flex1)107 cubic_log_hystart_event(struct cc_var *ccv, struct cubic *cubicd, uint8_t mod, uint32_t flex1)
108 {
109 /*
110 * Types of logs (mod value)
111 * 1 - rtt_thresh in flex1, checking to see if RTT is to great.
112 * 2 - rtt is too great, rtt_thresh in flex1.
113 * 3 - CSS is active incr in flex1
114 * 4 - A new round is beginning flex1 is round count
115 * 5 - A new RTT measurement flex1 is the new measurement.
116 * 6 - We enter CA ssthresh is also in flex1.
117 * 7 - Socket option to change hystart executed opt.val in flex1.
118 * 8 - Back out of CSS into SS, flex1 is the css_baseline_minrtt
119 * 9 - We enter CA, via an ECN mark.
120 * 10 - We enter CA, via a loss.
121 * 11 - We have slipped out of SS into CA via cwnd growth.
122 * 12 - After idle has re-enabled hystart++
123 */
124 struct tcpcb *tp;
125
126 if (hystart_bblogs == 0)
127 return;
128 tp = ccv->tp;
129 if (tcp_bblogging_on(tp)) {
130 union tcp_log_stackspecific log;
131 struct timeval tv;
132
133 memset(&log, 0, sizeof(log));
134 log.u_bbr.flex1 = flex1;
135 log.u_bbr.flex2 = cubicd->css_current_round_minrtt;
136 log.u_bbr.flex3 = cubicd->css_lastround_minrtt;
137 log.u_bbr.flex4 = cubicd->css_rttsample_count;
138 log.u_bbr.flex5 = cubicd->css_entered_at_round;
139 log.u_bbr.flex6 = cubicd->css_baseline_minrtt;
140 /* We only need bottom 16 bits of flags */
141 log.u_bbr.flex7 = cubicd->flags & 0x0000ffff;
142 log.u_bbr.flex8 = mod;
143 log.u_bbr.epoch = cubicd->css_current_round;
144 log.u_bbr.timeStamp = tcp_get_usecs(&tv);
145 log.u_bbr.lt_epoch = cubicd->css_fas_at_css_entry;
146 log.u_bbr.pkts_out = cubicd->css_last_fas;
147 log.u_bbr.delivered = cubicd->css_lowrtt_fas;
148 log.u_bbr.pkt_epoch = ccv->flags;
149 TCP_LOG_EVENTP(tp, NULL,
150 &tptosocket(tp)->so_rcv,
151 &tptosocket(tp)->so_snd,
152 TCP_HYSTART, 0,
153 0, &log, false, &tv);
154 }
155 }
156
157 static void
cubic_does_slow_start(struct cc_var * ccv,struct cubic * cubicd)158 cubic_does_slow_start(struct cc_var *ccv, struct cubic *cubicd)
159 {
160 /*
161 * In slow-start with ABC enabled and no RTO in sight?
162 * (Must not use abc_l_var > 1 if slow starting after
163 * an RTO. On RTO, snd_nxt = snd_una, so the
164 * snd_nxt == snd_max check is sufficient to
165 * handle this).
166 *
167 * XXXLAS: Find a way to signal SS after RTO that
168 * doesn't rely on tcpcb vars.
169 */
170 u_int cw = CCV(ccv, snd_cwnd);
171 uint32_t mss = tcp_fixed_maxseg(ccv->tp);
172 u_int incr = mss;
173 uint16_t abc_val;
174
175 cubicd->flags |= CUBICFLAG_IN_SLOWSTART;
176 if (ccv->flags & CCF_USE_LOCAL_ABC)
177 abc_val = ccv->labc;
178 else
179 abc_val = V_tcp_abc_l_var;
180 if ((ccv->flags & CCF_HYSTART_ALLOWED) &&
181 (cubicd->flags & CUBICFLAG_HYSTART_ENABLED) &&
182 ((cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) == 0)) {
183 /*
184 * Hystart is allowed and still enabled and we are not yet
185 * in CSS. Lets check to see if we can make a decision on
186 * if we need to go into CSS.
187 */
188 if ((cubicd->css_rttsample_count >= hystart_n_rttsamples) &&
189 (cubicd->css_current_round_minrtt != 0xffffffff) &&
190 (cubicd->css_lastround_minrtt != 0xffffffff)) {
191 uint32_t rtt_thresh;
192
193 /* Clamp (minrtt_thresh, lastround/8, maxrtt_thresh) */
194 rtt_thresh = (cubicd->css_lastround_minrtt >> 3);
195 if (rtt_thresh < hystart_minrtt_thresh)
196 rtt_thresh = hystart_minrtt_thresh;
197 if (rtt_thresh > hystart_maxrtt_thresh)
198 rtt_thresh = hystart_maxrtt_thresh;
199 cubic_log_hystart_event(ccv, cubicd, 1, rtt_thresh);
200
201 if (cubicd->css_current_round_minrtt >= (cubicd->css_lastround_minrtt + rtt_thresh)) {
202 /* Enter CSS */
203 cubicd->flags |= CUBICFLAG_HYSTART_IN_CSS;
204 cubicd->css_fas_at_css_entry = cubicd->css_lowrtt_fas;
205 /*
206 * The draft (v4) calls for us to set baseline to css_current_round_min
207 * but that can cause an oscillation. We probably shoudl be using
208 * css_lastround_minrtt, but the authors insist that will cause
209 * issues on exiting early. We will leave the draft version for now
210 * but I suspect this is incorrect.
211 */
212 cubicd->css_baseline_minrtt = cubicd->css_current_round_minrtt;
213 cubicd->css_entered_at_round = cubicd->css_current_round;
214 cubic_log_hystart_event(ccv, cubicd, 2, rtt_thresh);
215 }
216 }
217 }
218 if (CCV(ccv, snd_nxt) == CCV(ccv, snd_max))
219 incr = min(ccv->bytes_this_ack,
220 ccv->nsegs * abc_val * mss);
221 else
222 incr = min(ccv->bytes_this_ack, mss);
223
224 /* Only if Hystart is enabled will the flag get set */
225 if (cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) {
226 incr /= hystart_css_growth_div;
227 cubic_log_hystart_event(ccv, cubicd, 3, incr);
228 }
229 /* ABC is on by default, so incr equals 0 frequently. */
230 if (incr > 0)
231 CCV(ccv, snd_cwnd) = min((cw + incr),
232 TCP_MAXWIN << CCV(ccv, snd_scale));
233 }
234
235 static void
cubic_ack_received(struct cc_var * ccv,ccsignal_t type)236 cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
237 {
238 struct cubic *cubic_data;
239 uint32_t W_est, W_cubic, cwin, target, incr;
240 int usecs_since_epoch;
241 uint32_t mss = tcp_fixed_maxseg(ccv->tp);
242
243 cwin = CCV(ccv, snd_cwnd);
244 cubic_data = ccv->cc_data;
245 cubic_record_rtt(ccv);
246
247 /*
248 * For a regular ACK and we're not in cong/fast recovery and
249 * we're cwnd limited, always recalculate cwnd.
250 */
251 if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
252 (ccv->flags & CCF_CWND_LIMITED)) {
253 /* Use the logic in NewReno ack_received() for slow start. */
254 if (cwin <= CCV(ccv, snd_ssthresh) ||
255 cubic_data->min_rtt_usecs == TCPTV_SRTTBASE) {
256 cubic_does_slow_start(ccv, cubic_data);
257 } else {
258 if (cubic_data->flags & CUBICFLAG_HYSTART_IN_CSS) {
259 /*
260 * We have slipped into CA with
261 * CSS active. Deactivate all.
262 */
263 /* Turn off the CSS flag */
264 cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
265 /* Disable use of CSS in the future except long idle */
266 cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
267 cubic_log_hystart_event(ccv, cubic_data, 11, CCV(ccv, snd_ssthresh));
268 }
269 if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
270 CUBICFLAG_CONG_EVENT |
271 CUBICFLAG_IN_APPLIMIT)) {
272 /*
273 * At the beginning of the current congestion
274 * avoidance stage, The epoch variables
275 * (t_epoch, cwnd_epoch, K) are updated in the
276 * following three cases:
277 * 1) just exited the slow start
278 * 2) after a congestion event
279 * 3) application-limited
280 */
281 cubic_data->t_epoch = ticks;
282 cubic_data->cwnd_epoch = cwin;
283 cubic_data->K = cubic_k(cubic_data->W_max / mss,
284 cubic_data->cwnd_epoch / mss);
285 cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
286 CUBICFLAG_CONG_EVENT |
287 CUBICFLAG_IN_APPLIMIT);
288
289 if (cubic_data->flags & CUBICFLAG_RTO_EVENT) {
290 /* RFC9438 Section 4.8: Timeout */
291 cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
292 cubic_data->W_max = cwin;
293 cubic_data->K = 0;
294 }
295 }
296 usecs_since_epoch = (ticks - cubic_data->t_epoch) * tick;
297 if (usecs_since_epoch < 0) {
298 /*
299 * dragging t_epoch along
300 */
301 usecs_since_epoch = INT_MAX;
302 cubic_data->t_epoch = ticks - INT_MAX;
303 }
304 W_est = tf_cwnd(ccv);
305 /*
306 * The mean RTT is used to best reflect the equations.
307 */
308 W_cubic = cubic_cwnd(usecs_since_epoch +
309 cubic_data->mean_rtt_usecs,
310 cubic_data->W_max,
311 mss,
312 cubic_data->K);
313
314 if (W_cubic < W_est) {
315 /* RFC9438 Section 4.3: Reno-friendly region */
316 CCV(ccv, snd_cwnd) = W_est;
317 cubic_data->flags |= CUBICFLAG_IN_TF;
318 } else {
319 /*
320 * RFC9438 Section 4.4 or 4.5:
321 * Concave or Convex Region
322 */
323 if (W_cubic < cwin) {
324 target = cwin;
325 } else if (W_cubic > ((cwin * 3) >> 1)) {
326 target = (cwin * 3) >> 1;
327 } else {
328 target = W_cubic;
329 }
330 incr = (((target - cwin) << CUBIC_SHIFT) /
331 cwin * mss) >> CUBIC_SHIFT;
332 CCV(ccv, snd_cwnd) = cwin + incr;
333 }
334 }
335 } else if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
336 !(ccv->flags & CCF_CWND_LIMITED)) {
337 cubic_data->flags |= CUBICFLAG_IN_APPLIMIT;
338 }
339 }
340
341 /*
342 * This is a CUBIC specific implementation of after_idle.
343 * - Reset cwnd by calling New Reno implementation of after_idle.
344 * - Reset t_epoch.
345 */
346 static void
cubic_after_idle(struct cc_var * ccv)347 cubic_after_idle(struct cc_var *ccv)
348 {
349 struct cubic *cubic_data = ccv->cc_data;
350 uint32_t mss = tcp_fixed_maxseg(ccv->tp);
351
352 cubic_data->W_max = ulmax(cubic_data->W_max, CCV(ccv, snd_cwnd));
353 cubic_data->K = cubic_k(cubic_data->W_max / mss, cubic_data->cwnd_epoch / mss);
354 if ((cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) == 0) {
355 /*
356 * Re-enable hystart if we have been idle.
357 */
358 cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
359 cubic_data->flags |= CUBICFLAG_HYSTART_ENABLED;
360 cubic_log_hystart_event(ccv, cubic_data, 12, CCV(ccv, snd_ssthresh));
361 }
362 newreno_cc_after_idle(ccv);
363 cubic_data->t_epoch = ticks;
364 }
365
366 static void
cubic_cb_destroy(struct cc_var * ccv)367 cubic_cb_destroy(struct cc_var *ccv)
368 {
369 free(ccv->cc_data, M_CC_MEM);
370 }
371
372 static size_t
cubic_data_sz(void)373 cubic_data_sz(void)
374 {
375 return (sizeof(struct cubic));
376 }
377
378 static int
cubic_cb_init(struct cc_var * ccv,void * ptr)379 cubic_cb_init(struct cc_var *ccv, void *ptr)
380 {
381 struct cubic *cubic_data;
382
383 INP_WLOCK_ASSERT(tptoinpcb(ccv->tp));
384 if (ptr == NULL) {
385 cubic_data = malloc(sizeof(struct cubic), M_CC_MEM, M_NOWAIT|M_ZERO);
386 if (cubic_data == NULL)
387 return (ENOMEM);
388 } else
389 cubic_data = ptr;
390
391 /* Init some key variables with sensible defaults. */
392 cubic_data->t_epoch = 0;
393 cubic_data->cwnd_epoch = 0;
394 cubic_data->K = 0;
395 cubic_data->min_rtt_usecs = TCPTV_SRTTBASE;
396 cubic_data->mean_rtt_usecs = 1;
397
398 ccv->cc_data = cubic_data;
399 cubic_data->flags = CUBICFLAG_HYSTART_ENABLED;
400 /* At init set both to infinity */
401 cubic_data->css_lastround_minrtt = 0xffffffff;
402 cubic_data->css_current_round_minrtt = 0xffffffff;
403 cubic_data->css_current_round = 0;
404 cubic_data->css_baseline_minrtt = 0xffffffff;
405 cubic_data->css_rttsample_count = 0;
406 cubic_data->css_entered_at_round = 0;
407 cubic_data->css_fas_at_css_entry = 0;
408 cubic_data->css_lowrtt_fas = 0;
409 cubic_data->css_last_fas = 0;
410
411 return (0);
412 }
413
414 /*
415 * Perform any necessary tasks before we enter congestion recovery.
416 */
417 static void
cubic_cong_signal(struct cc_var * ccv,ccsignal_t type)418 cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
419 {
420 struct cubic *cubic_data;
421 uint32_t mss, pipe, ssthresh;
422
423 cubic_data = ccv->cc_data;
424 mss = tcp_fixed_maxseg(ccv->tp);
425
426 switch (type) {
427 case CC_NDUPACK:
428 if (cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) {
429 /* Make sure the flags are all off we had a loss */
430 cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
431 cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
432 cubic_log_hystart_event(ccv, cubic_data, 10, CCV(ccv, snd_ssthresh));
433 }
434 if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
435 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
436 ssthresh = cubic_get_ssthresh(ccv, mss);
437 CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
438 /*
439 * The congestion flag will recalculate K at the
440 * beginning of the congestion avoidance stage.
441 */
442 cubic_data->flags |= CUBICFLAG_CONG_EVENT;
443 }
444 ENTER_RECOVERY(CCV(ccv, t_flags));
445 }
446 break;
447
448 case CC_ECN:
449 if (cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) {
450 /* Make sure the flags are all off we had a loss */
451 cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
452 cubic_data->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
453 cubic_log_hystart_event(ccv, cubic_data, 9, CCV(ccv, snd_ssthresh));
454 }
455 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
456 ssthresh = cubic_get_ssthresh(ccv, mss);
457 CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
458 CCV(ccv, snd_cwnd) = max(ssthresh, mss);
459 /*
460 * The congestion flag will recalculate K at the
461 * beginning of the congestion avoidance stage.
462 */
463 cubic_data->flags |= CUBICFLAG_CONG_EVENT;
464 ENTER_CONGRECOVERY(CCV(ccv, t_flags));
465 }
466 break;
467
468 case CC_RTO:
469 /* RFC9438 Section 4.8: Timeout */
470 if (CCV(ccv, t_rxtshift) == 1) {
471 /*
472 * Remember the state only for the first RTO event. This
473 * will help us restore the state to the values seen
474 * at the most recent congestion avoidance stage before
475 * the current RTO event.
476 */
477 cubic_data->undo_t_epoch = cubic_data->t_epoch;
478 cubic_data->undo_cwnd_epoch = cubic_data->cwnd_epoch;
479 cubic_data->undo_W_max = cubic_data->W_max;
480 cubic_data->undo_K = cubic_data->K;
481 pipe = tcp_compute_pipe(ccv->tp);
482 CCV(ccv, snd_ssthresh) = max(2,
483 (((uint64_t)min(CCV(ccv, snd_wnd), pipe) *
484 CUBIC_BETA) >> CUBIC_SHIFT) / mss) * mss;
485 }
486 /*
487 * The RTO flag will recalculate K at the
488 * beginning of the congestion avoidance stage.
489 */
490 cubic_data->flags |= CUBICFLAG_RTO_EVENT;
491 CCV(ccv, snd_cwnd) = mss;
492 break;
493
494 case CC_RTO_ERR:
495 cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
496 cubic_data->K = cubic_data->undo_K;
497 cubic_data->W_max = cubic_data->undo_W_max;
498 cubic_data->cwnd_epoch = cubic_data->undo_cwnd_epoch;
499 cubic_data->t_epoch = cubic_data->undo_t_epoch;
500 break;
501 default:
502 break;
503 }
504 }
505
506 static void
cubic_conn_init(struct cc_var * ccv)507 cubic_conn_init(struct cc_var *ccv)
508 {
509 struct cubic *cubic_data;
510
511 cubic_data = ccv->cc_data;
512
513 /*
514 * Ensure we have a sane initial value for W_max recorded. Without
515 * this here bad things happen when entries from the TCP hostcache
516 * get used.
517 */
518 cubic_data->W_max = UINT_MAX;
519 }
520
521 static int
cubic_mod_init(void)522 cubic_mod_init(void)
523 {
524 return (0);
525 }
526
527 /*
528 * Perform any necessary tasks before we exit congestion recovery.
529 */
530 static void
cubic_post_recovery(struct cc_var * ccv)531 cubic_post_recovery(struct cc_var *ccv)
532 {
533 struct cubic *cubic_data;
534 int pipe;
535 uint32_t mss = tcp_fixed_maxseg(ccv->tp);
536
537 cubic_data = ccv->cc_data;
538 pipe = 0;
539
540 if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
541 /*
542 * If inflight data is less than ssthresh, set cwnd
543 * conservatively to avoid a burst of data, as suggested in
544 * the NewReno RFC. Otherwise, use the CUBIC method.
545 */
546 pipe = tcp_compute_pipe(ccv->tp);
547 if (pipe < CCV(ccv, snd_ssthresh))
548 /*
549 * Ensure that cwnd does not collapse to 1 MSS under
550 * adverse conditions. Implements RFC6582
551 */
552 CCV(ccv, snd_cwnd) = max(pipe, mss) + mss;
553 else
554 /* Update cwnd based on beta and adjusted W_max. */
555 CCV(ccv, snd_cwnd) = max(((uint64_t)cubic_data->W_max *
556 CUBIC_BETA) >> CUBIC_SHIFT,
557 2 * mss);
558 }
559
560 /* Calculate the average RTT between congestion epochs. */
561 if (cubic_data->epoch_ack_count > 0 &&
562 cubic_data->sum_rtt_usecs >= cubic_data->epoch_ack_count) {
563 cubic_data->mean_rtt_usecs = (int)(cubic_data->sum_rtt_usecs /
564 cubic_data->epoch_ack_count);
565 }
566
567 cubic_data->epoch_ack_count = 0;
568 cubic_data->sum_rtt_usecs = 0;
569 }
570
571 /*
572 * Record the min RTT and sum samples for the epoch average RTT calculation.
573 */
574 static void
cubic_record_rtt(struct cc_var * ccv)575 cubic_record_rtt(struct cc_var *ccv)
576 {
577 struct cubic *cubic_data;
578 uint32_t t_srtt_usecs;
579
580 /* Ignore srtt until a min number of samples have been taken. */
581 if (CCV(ccv, t_rttupdated) >= CUBIC_MIN_RTT_SAMPLES) {
582 cubic_data = ccv->cc_data;
583 t_srtt_usecs = tcp_get_srtt(ccv->tp,
584 TCP_TMR_GRANULARITY_USEC);
585 /*
586 * Record the current SRTT as our minrtt if it's the smallest
587 * we've seen or minrtt is currently equal to its initialised
588 * value.
589 *
590 * XXXLAS: Should there be some hysteresis for minrtt?
591 */
592 if ((t_srtt_usecs < cubic_data->min_rtt_usecs ||
593 cubic_data->min_rtt_usecs == TCPTV_SRTTBASE)) {
594 /* A minimal rtt is a single unshifted tick of a ticks
595 * timer. */
596 cubic_data->min_rtt_usecs = max(tick >> TCP_RTT_SHIFT,
597 t_srtt_usecs);
598
599 /*
600 * If the connection is within its first congestion
601 * epoch, ensure we prime mean_rtt_usecs with a
602 * reasonable value until the epoch average RTT is
603 * calculated in cubic_post_recovery().
604 */
605 if (cubic_data->min_rtt_usecs >
606 cubic_data->mean_rtt_usecs)
607 cubic_data->mean_rtt_usecs =
608 cubic_data->min_rtt_usecs;
609 }
610
611 /* Sum samples for epoch average RTT calculation. */
612 cubic_data->sum_rtt_usecs += t_srtt_usecs;
613 cubic_data->epoch_ack_count++;
614 }
615 }
616
617 /*
618 * Return the new value for ssthresh in the event of a congestion.
619 */
620 static uint32_t
cubic_get_ssthresh(struct cc_var * ccv,uint32_t maxseg)621 cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg)
622 {
623 struct cubic *cubic_data;
624 uint32_t cwnd, pipe;
625
626 cubic_data = ccv->cc_data;
627 cwnd = CCV(ccv, snd_cwnd);
628
629 /* RFC9438 Section 4.7: Fast convergence */
630 if (cwnd < cubic_data->W_max) {
631 cwnd = ((uint64_t)cwnd * CUBIC_FC_FACTOR) >> CUBIC_SHIFT;
632 }
633 cubic_data->W_max = cwnd;
634
635 if (cubic_data->flags & CUBICFLAG_IN_TF) {
636 /* If in the TCP friendly region, follow what newreno does. */
637 return (newreno_cc_cwnd_on_multiplicative_decrease(ccv, maxseg));
638
639 } else {
640 /*
641 * RFC9438 Section 4.6: Multiplicative Decrease
642 * Outside the TCP friendly region, set ssthresh to the size of
643 * inflight_size * beta.
644 */
645 pipe = tcp_compute_pipe(ccv->tp);
646 return ((pipe * CUBIC_BETA) >> CUBIC_SHIFT);
647 }
648 }
649
650 static void
cubic_rttsample(struct cc_var * ccv,uint32_t usec_rtt,uint32_t rxtcnt,uint32_t fas)651 cubic_rttsample(struct cc_var *ccv, uint32_t usec_rtt, uint32_t rxtcnt, uint32_t fas)
652 {
653 struct cubic *cubicd;
654
655 cubicd = ccv->cc_data;
656 if (rxtcnt > 1) {
657 /*
658 * Only look at RTT's that are non-ambiguous.
659 */
660 return;
661 }
662 cubicd->css_rttsample_count++;
663 cubicd->css_last_fas = fas;
664 if (cubicd->css_current_round_minrtt > usec_rtt) {
665 cubicd->css_current_round_minrtt = usec_rtt;
666 cubicd->css_lowrtt_fas = cubicd->css_last_fas;
667 }
668 if ((cubicd->css_rttsample_count >= hystart_n_rttsamples) &&
669 (cubicd->css_current_round_minrtt != 0xffffffff) &&
670 (cubicd->css_current_round_minrtt < cubicd->css_baseline_minrtt) &&
671 (cubicd->css_lastround_minrtt != 0xffffffff)) {
672 /*
673 * We were in CSS and the RTT is now less, we
674 * entered CSS erroneously.
675 */
676 cubicd->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
677 cubic_log_hystart_event(ccv, cubicd, 8, cubicd->css_baseline_minrtt);
678 cubicd->css_baseline_minrtt = 0xffffffff;
679 }
680 if (cubicd->flags & CUBICFLAG_HYSTART_ENABLED)
681 cubic_log_hystart_event(ccv, cubicd, 5, usec_rtt);
682 }
683
684 static void
cubic_newround(struct cc_var * ccv,uint32_t round_cnt)685 cubic_newround(struct cc_var *ccv, uint32_t round_cnt)
686 {
687 struct cubic *cubicd;
688
689 cubicd = ccv->cc_data;
690 /* We have entered a new round */
691 cubicd->css_lastround_minrtt = cubicd->css_current_round_minrtt;
692 cubicd->css_current_round_minrtt = 0xffffffff;
693 cubicd->css_rttsample_count = 0;
694 cubicd->css_current_round = round_cnt;
695 if ((cubicd->flags & CUBICFLAG_HYSTART_IN_CSS) &&
696 ((round_cnt - cubicd->css_entered_at_round) >= hystart_css_rounds)) {
697 /* Enter CA */
698 if (ccv->flags & CCF_HYSTART_CAN_SH_CWND) {
699 /*
700 * We engage more than snd_ssthresh, engage
701 * the brakes!! Though we will stay in SS to
702 * creep back up again, so lets leave CSS active
703 * and give us hystart_css_rounds more rounds.
704 */
705 if (ccv->flags & CCF_HYSTART_CONS_SSTH) {
706 CCV(ccv, snd_ssthresh) = ((cubicd->css_lowrtt_fas + cubicd->css_fas_at_css_entry) / 2);
707 } else {
708 CCV(ccv, snd_ssthresh) = cubicd->css_lowrtt_fas;
709 }
710 CCV(ccv, snd_cwnd) = cubicd->css_fas_at_css_entry;
711 cubicd->css_entered_at_round = round_cnt;
712 } else {
713 CCV(ccv, snd_ssthresh) = CCV(ccv, snd_cwnd);
714 /* Turn off the CSS flag */
715 cubicd->flags &= ~CUBICFLAG_HYSTART_IN_CSS;
716 /* Disable use of CSS in the future except long idle */
717 cubicd->flags &= ~CUBICFLAG_HYSTART_ENABLED;
718 }
719 cubic_log_hystart_event(ccv, cubicd, 6, CCV(ccv, snd_ssthresh));
720 }
721 if (cubicd->flags & CUBICFLAG_HYSTART_ENABLED)
722 cubic_log_hystart_event(ccv, cubicd, 4, round_cnt);
723 }
724
725 DECLARE_CC_MODULE(cubic, &cubic_cc_algo);
726 MODULE_VERSION(cubic, 2);
727