1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
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
8 * All rights reserved.
9 *
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.
14 *
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.
18 *
19 * Redistribution and use in source and binary forms, with or without
20 * modification, are permitted provided that the following conditions
21 * are met:
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.
27 *
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
38 * SUCH DAMAGE.
39 */
40
41 /*
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/
50 */
51
52 #include <sys/param.h>
53 #include <sys/kernel.h>
54 #include <sys/limits.h>
55 #include <sys/malloc.h>
56 #include <sys/module.h>
57 #include <sys/socket.h>
58 #include <sys/socketvar.h>
59 #include <sys/sysctl.h>
60 #include <sys/systm.h>
61
62 #include <net/vnet.h>
63
64 #include <net/route.h>
65 #include <net/route/nhop.h>
66
67 #include <netinet/in_pcb.h>
68 #include <netinet/tcp.h>
69 #include <netinet/tcp_seq.h>
70 #include <netinet/tcp_timer.h>
71 #include <netinet/tcp_var.h>
72 #include <netinet/cc/cc.h>
73 #include <netinet/cc/cc_module.h>
74
75 /* Fixed point math shifts. */
76 #define HTCP_SHIFT 8
77 #define HTCP_ALPHA_INC_SHIFT 4
78
79 #define HTCP_INIT_ALPHA 1
80 #define HTCP_DELTA_L hz /* 1 sec in ticks. */
81 #define HTCP_MINBETA 128 /* 0.5 << HTCP_SHIFT. */
82 #define HTCP_MAXBETA 204 /* ~0.8 << HTCP_SHIFT. */
83 #define HTCP_MINROWE 26 /* ~0.1 << HTCP_SHIFT. */
84 #define HTCP_MAXROWE 512 /* 2 << HTCP_SHIFT. */
85
86 /* RTT_ref (ms) used in the calculation of alpha if RTT scaling is enabled. */
87 #define HTCP_RTT_REF 100
88
89 /* Don't trust SRTT until this many samples have been taken. */
90 #define HTCP_MIN_RTT_SAMPLES 8
91
92 /*
93 * HTCP_CALC_ALPHA performs a fixed point math calculation to determine the
94 * value of alpha, based on the function defined in the HTCP spec.
95 *
96 * i.e. 1 + 10(delta - delta_l) + ((delta - delta_l) / 2) ^ 2
97 *
98 * "diff" is passed in to the macro as "delta - delta_l" and is expected to be
99 * in units of ticks.
100 *
101 * The joyousnous of fixed point maths means our function implementation looks a
102 * little funky...
103 *
104 * In order to maintain some precision in the calculations, a fixed point shift
105 * HTCP_ALPHA_INC_SHIFT is used to ensure the integer divisions don't
106 * truncate the results too badly.
107 *
108 * The "16" value is the "1" term in the alpha function shifted up by
109 * HTCP_ALPHA_INC_SHIFT
110 *
111 * The "160" value is the "10" multiplier in the alpha function multiplied by
112 * 2^HTCP_ALPHA_INC_SHIFT
113 *
114 * Specifying these as constants reduces the computations required. After
115 * up-shifting all the terms in the function and performing the required
116 * calculations, we down-shift the final result by HTCP_ALPHA_INC_SHIFT to
117 * ensure it is back in the correct range.
118 *
119 * The "hz" terms are required as kernels can be configured to run with
120 * different tick timers, which we have to adjust for in the alpha calculation
121 * (which originally was defined in terms of seconds).
122 *
123 * We also have to be careful to constrain the value of diff such that it won't
124 * overflow whilst performing the calculation. The middle term i.e. (160 * diff)
125 * / hz is the limiting factor in the calculation. We must constrain diff to be
126 * less than the max size of an int divided by the constant 160 figure
127 * i.e. diff < INT_MAX / 160
128 *
129 * NB: Changing HTCP_ALPHA_INC_SHIFT will require you to MANUALLY update the
130 * constants used in this function!
131 */
132 #define HTCP_CALC_ALPHA(diff) \
133 ((\
134 (16) + \
135 ((160 * (diff)) / hz) + \
136 (((diff) / hz) * (((diff) << HTCP_ALPHA_INC_SHIFT) / (4 * hz))) \
137 ) >> HTCP_ALPHA_INC_SHIFT)
138
139 static void htcp_ack_received(struct cc_var *ccv, ccsignal_t type);
140 static void htcp_cb_destroy(struct cc_var *ccv);
141 static int htcp_cb_init(struct cc_var *ccv, void *ptr);
142 static void htcp_cong_signal(struct cc_var *ccv, ccsignal_t type);
143 static int htcp_mod_init(void);
144 static void htcp_post_recovery(struct cc_var *ccv);
145 static void htcp_recalc_alpha(struct cc_var *ccv);
146 static void htcp_recalc_beta(struct cc_var *ccv);
147 static void htcp_record_rtt(struct cc_var *ccv);
148 static void htcp_ssthresh_update(struct cc_var *ccv);
149 static size_t htcp_data_sz(void);
150
151 struct htcp {
152 /* cwnd before entering cong recovery. */
153 unsigned long prev_cwnd;
154 /* cwnd additive increase parameter. */
155 int alpha;
156 /* cwnd multiplicative decrease parameter. */
157 int beta;
158 /* Largest rtt seen for the flow. */
159 int maxrtt;
160 /* Shortest rtt seen for the flow. */
161 int minrtt;
162 /* Time of last congestion event in ticks. */
163 int t_last_cong;
164 };
165
166 static int htcp_rtt_ref;
167 /*
168 * The maximum number of ticks the value of diff can reach in
169 * htcp_recalc_alpha() before alpha will stop increasing due to overflow.
170 * See comment above HTCP_CALC_ALPHA for more info.
171 */
172 static int htcp_max_diff = INT_MAX / ((1 << HTCP_ALPHA_INC_SHIFT) * 10);
173
174 /* Per-netstack vars. */
175 VNET_DEFINE_STATIC(u_int, htcp_adaptive_backoff) = 0;
176 VNET_DEFINE_STATIC(u_int, htcp_rtt_scaling) = 0;
177 #define V_htcp_adaptive_backoff VNET(htcp_adaptive_backoff)
178 #define V_htcp_rtt_scaling VNET(htcp_rtt_scaling)
179
180 struct cc_algo htcp_cc_algo = {
181 .name = "htcp",
182 .ack_received = htcp_ack_received,
183 .cb_destroy = htcp_cb_destroy,
184 .cb_init = htcp_cb_init,
185 .cong_signal = htcp_cong_signal,
186 .mod_init = htcp_mod_init,
187 .post_recovery = htcp_post_recovery,
188 .cc_data_sz = htcp_data_sz,
189 .after_idle = newreno_cc_after_idle,
190 };
191
192 static void
htcp_ack_received(struct cc_var * ccv,ccsignal_t type)193 htcp_ack_received(struct cc_var *ccv, ccsignal_t type)
194 {
195 struct htcp *htcp_data;
196
197 htcp_data = ccv->cc_data;
198 htcp_record_rtt(ccv);
199
200 /*
201 * Regular ACK and we're not in cong/fast recovery and we're cwnd
202 * limited and we're either not doing ABC or are slow starting or are
203 * doing ABC and we've sent a cwnd's worth of bytes.
204 */
205 if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
206 (ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
207 CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
208 (V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
209 htcp_recalc_beta(ccv);
210 htcp_recalc_alpha(ccv);
211 /*
212 * Use the logic in NewReno ack_received() for slow start and
213 * for the first HTCP_DELTA_L ticks after either the flow starts
214 * or a congestion event (when alpha equals 1).
215 */
216 if (htcp_data->alpha == 1 ||
217 CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh))
218 newreno_cc_ack_received(ccv, type);
219 else {
220 if (V_tcp_do_rfc3465) {
221 /* Increment cwnd by alpha segments. */
222 CCV(ccv, snd_cwnd) += htcp_data->alpha *
223 CCV(ccv, t_maxseg);
224 ccv->flags &= ~CCF_ABC_SENTAWND;
225 } else
226 /*
227 * Increment cwnd by alpha/cwnd segments to
228 * approximate an increase of alpha segments
229 * per RTT.
230 */
231 CCV(ccv, snd_cwnd) += (((htcp_data->alpha <<
232 HTCP_SHIFT) / (max(1,
233 CCV(ccv, snd_cwnd) / CCV(ccv, t_maxseg)))) *
234 CCV(ccv, t_maxseg)) >> HTCP_SHIFT;
235 }
236 }
237 }
238
239 static void
htcp_cb_destroy(struct cc_var * ccv)240 htcp_cb_destroy(struct cc_var *ccv)
241 {
242 free(ccv->cc_data, M_CC_MEM);
243 }
244
245 static size_t
htcp_data_sz(void)246 htcp_data_sz(void)
247 {
248 return(sizeof(struct htcp));
249 }
250
251 static int
htcp_cb_init(struct cc_var * ccv,void * ptr)252 htcp_cb_init(struct cc_var *ccv, void *ptr)
253 {
254 struct htcp *htcp_data;
255
256 INP_WLOCK_ASSERT(tptoinpcb(ccv->ccvc.tcp));
257 if (ptr == NULL) {
258 htcp_data = malloc(sizeof(struct htcp), M_CC_MEM, M_NOWAIT);
259 if (htcp_data == NULL)
260 return (ENOMEM);
261 } else
262 htcp_data = ptr;
263
264 /* Init some key variables with sensible defaults. */
265 htcp_data->alpha = HTCP_INIT_ALPHA;
266 htcp_data->beta = HTCP_MINBETA;
267 htcp_data->maxrtt = TCPTV_SRTTBASE;
268 htcp_data->minrtt = TCPTV_SRTTBASE;
269 htcp_data->prev_cwnd = 0;
270 htcp_data->t_last_cong = ticks;
271
272 ccv->cc_data = htcp_data;
273
274 return (0);
275 }
276
277 /*
278 * Perform any necessary tasks before we enter congestion recovery.
279 */
280 static void
htcp_cong_signal(struct cc_var * ccv,ccsignal_t type)281 htcp_cong_signal(struct cc_var *ccv, ccsignal_t type)
282 {
283 struct htcp *htcp_data;
284 uint32_t mss, pipe;
285
286 htcp_data = ccv->cc_data;
287 mss = tcp_fixed_maxseg(ccv->ccvc.tcp);
288
289 switch (type) {
290 case CC_NDUPACK:
291 if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
292 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
293 /*
294 * Apply hysteresis to maxrtt to ensure
295 * reductions in the RTT are reflected in our
296 * measurements.
297 */
298 htcp_data->maxrtt = (htcp_data->minrtt +
299 (htcp_data->maxrtt - htcp_data->minrtt) *
300 95) / 100;
301 htcp_ssthresh_update(ccv);
302 htcp_data->t_last_cong = ticks;
303 htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
304 }
305 ENTER_RECOVERY(CCV(ccv, t_flags));
306 }
307 break;
308
309 case CC_ECN:
310 if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
311 /*
312 * Apply hysteresis to maxrtt to ensure reductions in
313 * the RTT are reflected in our measurements.
314 */
315 htcp_data->maxrtt = (htcp_data->minrtt + (htcp_data->maxrtt -
316 htcp_data->minrtt) * 95) / 100;
317 htcp_ssthresh_update(ccv);
318 CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
319 htcp_data->t_last_cong = ticks;
320 htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
321 ENTER_CONGRECOVERY(CCV(ccv, t_flags));
322 }
323 break;
324
325 case CC_RTO:
326 if (CCV(ccv, t_rxtshift) == 1) {
327 if (V_tcp_do_newsack) {
328 pipe = tcp_compute_pipe(ccv->ccvc.tcp);
329 } else {
330 pipe = CCV(ccv, snd_max) -
331 CCV(ccv, snd_fack) +
332 CCV(ccv, sackhint.sack_bytes_rexmit);
333 }
334 CCV(ccv, snd_ssthresh) = max(2,
335 min(CCV(ccv, snd_wnd), pipe) / 2 / mss) * mss;
336 }
337 CCV(ccv, snd_cwnd) = mss;
338 /*
339 * Grab the current time and record it so we know when the
340 * most recent congestion event was. Only record it when the
341 * timeout has fired more than once, as there is a reasonable
342 * chance the first one is a false alarm and may not indicate
343 * congestion.
344 */
345 if (CCV(ccv, t_rxtshift) >= 2)
346 htcp_data->t_last_cong = ticks;
347 break;
348 default:
349 break;
350 }
351 }
352
353 static int
htcp_mod_init(void)354 htcp_mod_init(void)
355 {
356 /*
357 * HTCP_RTT_REF is defined in ms, and t_srtt in the tcpcb is stored in
358 * units of TCP_RTT_SCALE*hz. Scale HTCP_RTT_REF to be in the same units
359 * as t_srtt.
360 */
361 htcp_rtt_ref = (HTCP_RTT_REF * TCP_RTT_SCALE * hz) / 1000;
362 return (0);
363 }
364
365 /*
366 * Perform any necessary tasks before we exit congestion recovery.
367 */
368 static void
htcp_post_recovery(struct cc_var * ccv)369 htcp_post_recovery(struct cc_var *ccv)
370 {
371 int pipe;
372 struct htcp *htcp_data;
373
374 pipe = 0;
375 htcp_data = ccv->cc_data;
376
377 if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
378 /*
379 * If inflight data is less than ssthresh, set cwnd
380 * conservatively to avoid a burst of data, as suggested in the
381 * NewReno RFC. Otherwise, use the HTCP method.
382 *
383 * XXXLAS: Find a way to do this without needing curack
384 */
385 if (V_tcp_do_newsack)
386 pipe = tcp_compute_pipe(ccv->ccvc.tcp);
387 else
388 pipe = CCV(ccv, snd_max) - ccv->curack;
389
390 if (pipe < CCV(ccv, snd_ssthresh))
391 /*
392 * Ensure that cwnd down not collape to 1 MSS under
393 * adverse conditions. Implements RFC6582
394 */
395 CCV(ccv, snd_cwnd) = max(pipe, CCV(ccv, t_maxseg)) +
396 CCV(ccv, t_maxseg);
397 else
398 CCV(ccv, snd_cwnd) = max(1, ((htcp_data->beta *
399 htcp_data->prev_cwnd / CCV(ccv, t_maxseg))
400 >> HTCP_SHIFT)) * CCV(ccv, t_maxseg);
401 }
402 }
403
404 static void
htcp_recalc_alpha(struct cc_var * ccv)405 htcp_recalc_alpha(struct cc_var *ccv)
406 {
407 struct htcp *htcp_data;
408 int alpha, diff, now;
409
410 htcp_data = ccv->cc_data;
411 now = ticks;
412
413 /*
414 * If ticks has wrapped around (will happen approximately once every 49
415 * days on a machine with the default kern.hz=1000) and a flow straddles
416 * the wrap point, our alpha calcs will be completely wrong. We cut our
417 * losses and restart alpha from scratch by setting t_last_cong = now -
418 * HTCP_DELTA_L.
419 *
420 * This does not deflate our cwnd at all. It simply slows the rate cwnd
421 * is growing by until alpha regains the value it held prior to taking
422 * this drastic measure.
423 */
424 if (now < htcp_data->t_last_cong)
425 htcp_data->t_last_cong = now - HTCP_DELTA_L;
426
427 diff = now - htcp_data->t_last_cong - HTCP_DELTA_L;
428
429 /* Cap alpha if the value of diff would overflow HTCP_CALC_ALPHA(). */
430 if (diff < htcp_max_diff) {
431 /*
432 * If it has been more than HTCP_DELTA_L ticks since congestion,
433 * increase alpha according to the function defined in the spec.
434 */
435 if (diff > 0) {
436 alpha = HTCP_CALC_ALPHA(diff);
437
438 /*
439 * Adaptive backoff fairness adjustment:
440 * 2 * (1 - beta) * alpha_raw
441 */
442 if (V_htcp_adaptive_backoff)
443 alpha = max(1, (2 * ((1 << HTCP_SHIFT) -
444 htcp_data->beta) * alpha) >> HTCP_SHIFT);
445
446 /*
447 * RTT scaling: (RTT / RTT_ref) * alpha
448 * alpha will be the raw value from HTCP_CALC_ALPHA() if
449 * adaptive backoff is off, or the adjusted value if
450 * adaptive backoff is on.
451 */
452 if (V_htcp_rtt_scaling)
453 alpha = max(1, (min(max(HTCP_MINROWE,
454 (tcp_get_srtt(ccv->ccvc.tcp, TCP_TMR_GRANULARITY_TICKS) << HTCP_SHIFT) /
455 htcp_rtt_ref), HTCP_MAXROWE) * alpha)
456 >> HTCP_SHIFT);
457
458 } else
459 alpha = 1;
460
461 htcp_data->alpha = alpha;
462 }
463 }
464
465 static void
htcp_recalc_beta(struct cc_var * ccv)466 htcp_recalc_beta(struct cc_var *ccv)
467 {
468 struct htcp *htcp_data;
469
470 htcp_data = ccv->cc_data;
471
472 /*
473 * TCPTV_SRTTBASE is the initialised value of each connection's SRTT, so
474 * we only calc beta if the connection's SRTT has been changed from its
475 * initial value. beta is bounded to ensure it is always between
476 * HTCP_MINBETA and HTCP_MAXBETA.
477 */
478 if (V_htcp_adaptive_backoff && htcp_data->minrtt != TCPTV_SRTTBASE &&
479 htcp_data->maxrtt != TCPTV_SRTTBASE)
480 htcp_data->beta = min(max(HTCP_MINBETA,
481 (htcp_data->minrtt << HTCP_SHIFT) / htcp_data->maxrtt),
482 HTCP_MAXBETA);
483 else
484 htcp_data->beta = HTCP_MINBETA;
485 }
486
487 /*
488 * Record the minimum and maximum RTT seen for the connection. These are used in
489 * the calculation of beta if adaptive backoff is enabled.
490 */
491 static void
htcp_record_rtt(struct cc_var * ccv)492 htcp_record_rtt(struct cc_var *ccv)
493 {
494 struct htcp *htcp_data;
495
496 htcp_data = ccv->cc_data;
497
498 /* XXXLAS: Should there be some hysteresis for minrtt? */
499
500 /*
501 * Record the current SRTT as our minrtt if it's the smallest we've seen
502 * or minrtt is currently equal to its initialised value. Ignore SRTT
503 * until a min number of samples have been taken.
504 */
505 if ((tcp_get_srtt(ccv->ccvc.tcp, TCP_TMR_GRANULARITY_TICKS) < htcp_data->minrtt ||
506 htcp_data->minrtt == TCPTV_SRTTBASE) &&
507 (CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES))
508 htcp_data->minrtt = tcp_get_srtt(ccv->ccvc.tcp, TCP_TMR_GRANULARITY_TICKS);
509
510 /*
511 * Record the current SRTT as our maxrtt if it's the largest we've
512 * seen. Ignore SRTT until a min number of samples have been taken.
513 */
514 if (tcp_get_srtt(ccv->ccvc.tcp, TCP_TMR_GRANULARITY_TICKS) > htcp_data->maxrtt
515 && CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES)
516 htcp_data->maxrtt = tcp_get_srtt(ccv->ccvc.tcp, TCP_TMR_GRANULARITY_TICKS);
517 }
518
519 /*
520 * Update the ssthresh in the event of congestion.
521 */
522 static void
htcp_ssthresh_update(struct cc_var * ccv)523 htcp_ssthresh_update(struct cc_var *ccv)
524 {
525 struct htcp *htcp_data;
526
527 htcp_data = ccv->cc_data;
528
529 /*
530 * On the first congestion event, set ssthresh to cwnd * 0.5, on
531 * subsequent congestion events, set it to cwnd * beta.
532 */
533 if (CCV(ccv, snd_ssthresh) == TCP_MAXWIN << TCP_MAX_WINSHIFT)
534 CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
535 HTCP_MINBETA) >> HTCP_SHIFT;
536 else {
537 htcp_recalc_beta(ccv);
538 CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
539 htcp_data->beta) >> HTCP_SHIFT;
540 }
541 }
542
543 SYSCTL_DECL(_net_inet_tcp_cc_htcp);
544 SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, htcp, CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
545 "H-TCP related settings");
546 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, adaptive_backoff,
547 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_adaptive_backoff), 0,
548 "enable H-TCP adaptive backoff");
549 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, rtt_scaling,
550 CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_rtt_scaling), 0,
551 "enable H-TCP RTT scaling");
552
553 DECLARE_CC_MODULE(htcp, &htcp_cc_algo);
554 MODULE_VERSION(htcp, 2);
555