2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
4 * Copyright (c) 2007-2008
5 * Swinburne University of Technology, Melbourne, Australia
6 * Copyright (c) 2009-2010 Lawrence Stewart <lstewart@freebsd.org>
7 * Copyright (c) 2010 The FreeBSD Foundation
10 * This software was developed at the Centre for Advanced Internet
11 * Architectures, Swinburne University of Technology, by Lawrence Stewart and
12 * James Healy, made possible in part by a grant from the Cisco University
13 * Research Program Fund at Community Foundation Silicon Valley.
15 * Portions of this software were developed at the Centre for Advanced
16 * Internet Architectures, Swinburne University of Technology, Melbourne,
17 * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
19 * Redistribution and use in source and binary forms, with or without
20 * modification, are permitted provided that the following conditions
22 * 1. Redistributions of source code must retain the above copyright
23 * notice, this list of conditions and the following disclaimer.
24 * 2. Redistributions in binary form must reproduce the above copyright
25 * notice, this list of conditions and the following disclaimer in the
26 * documentation and/or other materials provided with the distribution.
28 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
29 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
30 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
31 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
32 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
33 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
34 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
35 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
36 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
37 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
42 * An implementation of the H-TCP congestion control algorithm for FreeBSD,
43 * based on the Internet Draft "draft-leith-tcp-htcp-06.txt" by Leith and
44 * Shorten. Originally released as part of the NewTCP research project at
45 * Swinburne University of Technology's Centre for Advanced Internet
46 * Architectures, Melbourne, Australia, which was made possible in part by a
47 * grant from the Cisco University Research Program Fund at Community Foundation
48 * Silicon Valley. More details are available at:
49 * http://caia.swin.edu.au/urp/newtcp/
52 #include <sys/cdefs.h>
53 __FBSDID("$FreeBSD$");
55 #include <sys/param.h>
56 #include <sys/kernel.h>
57 #include <sys/limits.h>
58 #include <sys/malloc.h>
59 #include <sys/module.h>
60 #include <sys/socket.h>
61 #include <sys/socketvar.h>
62 #include <sys/sysctl.h>
63 #include <sys/systm.h>
67 #include <netinet/tcp.h>
68 #include <netinet/tcp_seq.h>
69 #include <netinet/tcp_timer.h>
70 #include <netinet/tcp_var.h>
71 #include <netinet/cc/cc.h>
72 #include <netinet/cc/cc_module.h>
74 /* Fixed point math shifts. */
76 #define HTCP_ALPHA_INC_SHIFT 4
78 #define HTCP_INIT_ALPHA 1
79 #define HTCP_DELTA_L hz /* 1 sec in ticks. */
80 #define HTCP_MINBETA 128 /* 0.5 << HTCP_SHIFT. */
81 #define HTCP_MAXBETA 204 /* ~0.8 << HTCP_SHIFT. */
82 #define HTCP_MINROWE 26 /* ~0.1 << HTCP_SHIFT. */
83 #define HTCP_MAXROWE 512 /* 2 << HTCP_SHIFT. */
85 /* RTT_ref (ms) used in the calculation of alpha if RTT scaling is enabled. */
86 #define HTCP_RTT_REF 100
88 /* Don't trust SRTT until this many samples have been taken. */
89 #define HTCP_MIN_RTT_SAMPLES 8
92 * HTCP_CALC_ALPHA performs a fixed point math calculation to determine the
93 * value of alpha, based on the function defined in the HTCP spec.
95 * i.e. 1 + 10(delta - delta_l) + ((delta - delta_l) / 2) ^ 2
97 * "diff" is passed in to the macro as "delta - delta_l" and is expected to be
100 * The joyousnous of fixed point maths means our function implementation looks a
103 * In order to maintain some precision in the calculations, a fixed point shift
104 * HTCP_ALPHA_INC_SHIFT is used to ensure the integer divisions don't
105 * truncate the results too badly.
107 * The "16" value is the "1" term in the alpha function shifted up by
108 * HTCP_ALPHA_INC_SHIFT
110 * The "160" value is the "10" multiplier in the alpha function multiplied by
111 * 2^HTCP_ALPHA_INC_SHIFT
113 * Specifying these as constants reduces the computations required. After
114 * up-shifting all the terms in the function and performing the required
115 * calculations, we down-shift the final result by HTCP_ALPHA_INC_SHIFT to
116 * ensure it is back in the correct range.
118 * The "hz" terms are required as kernels can be configured to run with
119 * different tick timers, which we have to adjust for in the alpha calculation
120 * (which originally was defined in terms of seconds).
122 * We also have to be careful to constrain the value of diff such that it won't
123 * overflow whilst performing the calculation. The middle term i.e. (160 * diff)
124 * / hz is the limiting factor in the calculation. We must constrain diff to be
125 * less than the max size of an int divided by the constant 160 figure
126 * i.e. diff < INT_MAX / 160
128 * NB: Changing HTCP_ALPHA_INC_SHIFT will require you to MANUALLY update the
129 * constants used in this function!
131 #define HTCP_CALC_ALPHA(diff) \
134 ((160 * (diff)) / hz) + \
135 (((diff) / hz) * (((diff) << HTCP_ALPHA_INC_SHIFT) / (4 * hz))) \
136 ) >> HTCP_ALPHA_INC_SHIFT)
138 static void htcp_ack_received(struct cc_var *ccv, uint16_t type);
139 static void htcp_cb_destroy(struct cc_var *ccv);
140 static int htcp_cb_init(struct cc_var *ccv);
141 static void htcp_cong_signal(struct cc_var *ccv, uint32_t type);
142 static int htcp_mod_init(void);
143 static void htcp_post_recovery(struct cc_var *ccv);
144 static void htcp_recalc_alpha(struct cc_var *ccv);
145 static void htcp_recalc_beta(struct cc_var *ccv);
146 static void htcp_record_rtt(struct cc_var *ccv);
147 static void htcp_ssthresh_update(struct cc_var *ccv);
150 /* cwnd before entering cong recovery. */
151 unsigned long prev_cwnd;
152 /* cwnd additive increase parameter. */
154 /* cwnd multiplicative decrease parameter. */
156 /* Largest rtt seen for the flow. */
158 /* Shortest rtt seen for the flow. */
160 /* Time of last congestion event in ticks. */
164 static int htcp_rtt_ref;
166 * The maximum number of ticks the value of diff can reach in
167 * htcp_recalc_alpha() before alpha will stop increasing due to overflow.
168 * See comment above HTCP_CALC_ALPHA for more info.
170 static int htcp_max_diff = INT_MAX / ((1 << HTCP_ALPHA_INC_SHIFT) * 10);
172 /* Per-netstack vars. */
173 VNET_DEFINE_STATIC(u_int, htcp_adaptive_backoff) = 0;
174 VNET_DEFINE_STATIC(u_int, htcp_rtt_scaling) = 0;
175 #define V_htcp_adaptive_backoff VNET(htcp_adaptive_backoff)
176 #define V_htcp_rtt_scaling VNET(htcp_rtt_scaling)
178 static MALLOC_DEFINE(M_HTCP, "htcp data",
179 "Per connection data required for the HTCP congestion control algorithm");
181 struct cc_algo htcp_cc_algo = {
183 .ack_received = htcp_ack_received,
184 .cb_destroy = htcp_cb_destroy,
185 .cb_init = htcp_cb_init,
186 .cong_signal = htcp_cong_signal,
187 .mod_init = htcp_mod_init,
188 .post_recovery = htcp_post_recovery,
192 htcp_ack_received(struct cc_var *ccv, uint16_t type)
194 struct htcp *htcp_data;
196 htcp_data = ccv->cc_data;
197 htcp_record_rtt(ccv);
200 * Regular ACK and we're not in cong/fast recovery and we're cwnd
201 * limited and we're either not doing ABC or are slow starting or are
202 * doing ABC and we've sent a cwnd's worth of bytes.
204 if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
205 (ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
206 CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
207 (V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
208 htcp_recalc_beta(ccv);
209 htcp_recalc_alpha(ccv);
211 * Use the logic in NewReno ack_received() for slow start and
212 * for the first HTCP_DELTA_L ticks after either the flow starts
213 * or a congestion event (when alpha equals 1).
215 if (htcp_data->alpha == 1 ||
216 CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh))
217 newreno_cc_algo.ack_received(ccv, type);
219 if (V_tcp_do_rfc3465) {
220 /* Increment cwnd by alpha segments. */
221 CCV(ccv, snd_cwnd) += htcp_data->alpha *
223 ccv->flags &= ~CCF_ABC_SENTAWND;
226 * Increment cwnd by alpha/cwnd segments to
227 * approximate an increase of alpha segments
230 CCV(ccv, snd_cwnd) += (((htcp_data->alpha <<
231 HTCP_SHIFT) / (CCV(ccv, snd_cwnd) /
232 CCV(ccv, t_maxseg))) * CCV(ccv, t_maxseg))
239 htcp_cb_destroy(struct cc_var *ccv)
241 free(ccv->cc_data, M_HTCP);
245 htcp_cb_init(struct cc_var *ccv)
247 struct htcp *htcp_data;
249 htcp_data = malloc(sizeof(struct htcp), M_HTCP, M_NOWAIT);
251 if (htcp_data == NULL)
254 /* Init some key variables with sensible defaults. */
255 htcp_data->alpha = HTCP_INIT_ALPHA;
256 htcp_data->beta = HTCP_MINBETA;
257 htcp_data->maxrtt = TCPTV_SRTTBASE;
258 htcp_data->minrtt = TCPTV_SRTTBASE;
259 htcp_data->prev_cwnd = 0;
260 htcp_data->t_last_cong = ticks;
262 ccv->cc_data = htcp_data;
268 * Perform any necessary tasks before we enter congestion recovery.
271 htcp_cong_signal(struct cc_var *ccv, uint32_t type)
273 struct htcp *htcp_data;
275 htcp_data = ccv->cc_data;
279 if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
280 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
282 * Apply hysteresis to maxrtt to ensure
283 * reductions in the RTT are reflected in our
286 htcp_data->maxrtt = (htcp_data->minrtt +
287 (htcp_data->maxrtt - htcp_data->minrtt) *
289 htcp_ssthresh_update(ccv);
290 htcp_data->t_last_cong = ticks;
291 htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
293 ENTER_RECOVERY(CCV(ccv, t_flags));
298 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
300 * Apply hysteresis to maxrtt to ensure reductions in
301 * the RTT are reflected in our measurements.
303 htcp_data->maxrtt = (htcp_data->minrtt + (htcp_data->maxrtt -
304 htcp_data->minrtt) * 95) / 100;
305 htcp_ssthresh_update(ccv);
306 CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
307 htcp_data->t_last_cong = ticks;
308 htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
309 ENTER_CONGRECOVERY(CCV(ccv, t_flags));
315 * Grab the current time and record it so we know when the
316 * most recent congestion event was. Only record it when the
317 * timeout has fired more than once, as there is a reasonable
318 * chance the first one is a false alarm and may not indicate
321 if (CCV(ccv, t_rxtshift) >= 2)
322 htcp_data->t_last_cong = ticks;
331 htcp_cc_algo.after_idle = newreno_cc_algo.after_idle;
334 * HTCP_RTT_REF is defined in ms, and t_srtt in the tcpcb is stored in
335 * units of TCP_RTT_SCALE*hz. Scale HTCP_RTT_REF to be in the same units
338 htcp_rtt_ref = (HTCP_RTT_REF * TCP_RTT_SCALE * hz) / 1000;
344 * Perform any necessary tasks before we exit congestion recovery.
347 htcp_post_recovery(struct cc_var *ccv)
350 struct htcp *htcp_data;
353 htcp_data = ccv->cc_data;
355 if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
357 * If inflight data is less than ssthresh, set cwnd
358 * conservatively to avoid a burst of data, as suggested in the
359 * NewReno RFC. Otherwise, use the HTCP method.
361 * XXXLAS: Find a way to do this without needing curack
363 if (V_tcp_do_rfc6675_pipe)
364 pipe = tcp_compute_pipe(ccv->ccvc.tcp);
366 pipe = CCV(ccv, snd_max) - ccv->curack;
368 if (pipe < CCV(ccv, snd_ssthresh))
369 CCV(ccv, snd_cwnd) = pipe + CCV(ccv, t_maxseg);
371 CCV(ccv, snd_cwnd) = max(1, ((htcp_data->beta *
372 htcp_data->prev_cwnd / CCV(ccv, t_maxseg))
373 >> HTCP_SHIFT)) * CCV(ccv, t_maxseg);
378 htcp_recalc_alpha(struct cc_var *ccv)
380 struct htcp *htcp_data;
381 int alpha, diff, now;
383 htcp_data = ccv->cc_data;
387 * If ticks has wrapped around (will happen approximately once every 49
388 * days on a machine with the default kern.hz=1000) and a flow straddles
389 * the wrap point, our alpha calcs will be completely wrong. We cut our
390 * losses and restart alpha from scratch by setting t_last_cong = now -
393 * This does not deflate our cwnd at all. It simply slows the rate cwnd
394 * is growing by until alpha regains the value it held prior to taking
395 * this drastic measure.
397 if (now < htcp_data->t_last_cong)
398 htcp_data->t_last_cong = now - HTCP_DELTA_L;
400 diff = now - htcp_data->t_last_cong - HTCP_DELTA_L;
402 /* Cap alpha if the value of diff would overflow HTCP_CALC_ALPHA(). */
403 if (diff < htcp_max_diff) {
405 * If it has been more than HTCP_DELTA_L ticks since congestion,
406 * increase alpha according to the function defined in the spec.
409 alpha = HTCP_CALC_ALPHA(diff);
412 * Adaptive backoff fairness adjustment:
413 * 2 * (1 - beta) * alpha_raw
415 if (V_htcp_adaptive_backoff)
416 alpha = max(1, (2 * ((1 << HTCP_SHIFT) -
417 htcp_data->beta) * alpha) >> HTCP_SHIFT);
420 * RTT scaling: (RTT / RTT_ref) * alpha
421 * alpha will be the raw value from HTCP_CALC_ALPHA() if
422 * adaptive backoff is off, or the adjusted value if
423 * adaptive backoff is on.
425 if (V_htcp_rtt_scaling)
426 alpha = max(1, (min(max(HTCP_MINROWE,
427 (CCV(ccv, t_srtt) << HTCP_SHIFT) /
428 htcp_rtt_ref), HTCP_MAXROWE) * alpha)
434 htcp_data->alpha = alpha;
439 htcp_recalc_beta(struct cc_var *ccv)
441 struct htcp *htcp_data;
443 htcp_data = ccv->cc_data;
446 * TCPTV_SRTTBASE is the initialised value of each connection's SRTT, so
447 * we only calc beta if the connection's SRTT has been changed from its
448 * initial value. beta is bounded to ensure it is always between
449 * HTCP_MINBETA and HTCP_MAXBETA.
451 if (V_htcp_adaptive_backoff && htcp_data->minrtt != TCPTV_SRTTBASE &&
452 htcp_data->maxrtt != TCPTV_SRTTBASE)
453 htcp_data->beta = min(max(HTCP_MINBETA,
454 (htcp_data->minrtt << HTCP_SHIFT) / htcp_data->maxrtt),
457 htcp_data->beta = HTCP_MINBETA;
461 * Record the minimum and maximum RTT seen for the connection. These are used in
462 * the calculation of beta if adaptive backoff is enabled.
465 htcp_record_rtt(struct cc_var *ccv)
467 struct htcp *htcp_data;
469 htcp_data = ccv->cc_data;
471 /* XXXLAS: Should there be some hysteresis for minrtt? */
474 * Record the current SRTT as our minrtt if it's the smallest we've seen
475 * or minrtt is currently equal to its initialised value. Ignore SRTT
476 * until a min number of samples have been taken.
478 if ((CCV(ccv, t_srtt) < htcp_data->minrtt ||
479 htcp_data->minrtt == TCPTV_SRTTBASE) &&
480 (CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES))
481 htcp_data->minrtt = CCV(ccv, t_srtt);
484 * Record the current SRTT as our maxrtt if it's the largest we've
485 * seen. Ignore SRTT until a min number of samples have been taken.
487 if (CCV(ccv, t_srtt) > htcp_data->maxrtt
488 && CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES)
489 htcp_data->maxrtt = CCV(ccv, t_srtt);
493 * Update the ssthresh in the event of congestion.
496 htcp_ssthresh_update(struct cc_var *ccv)
498 struct htcp *htcp_data;
500 htcp_data = ccv->cc_data;
503 * On the first congestion event, set ssthresh to cwnd * 0.5, on
504 * subsequent congestion events, set it to cwnd * beta.
506 if (CCV(ccv, snd_ssthresh) == TCP_MAXWIN << TCP_MAX_WINSHIFT)
507 CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
508 HTCP_MINBETA) >> HTCP_SHIFT;
510 htcp_recalc_beta(ccv);
511 CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
512 htcp_data->beta) >> HTCP_SHIFT;
517 SYSCTL_DECL(_net_inet_tcp_cc_htcp);
518 SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, htcp, CTLFLAG_RW,
519 NULL, "H-TCP related settings");
520 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, adaptive_backoff,
521 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_adaptive_backoff), 0,
522 "enable H-TCP adaptive backoff");
523 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, rtt_scaling,
524 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_rtt_scaling), 0,
525 "enable H-TCP RTT scaling");
527 DECLARE_CC_MODULE(htcp, &htcp_cc_algo);