xref: /freebsd/sys/netinet/cc/cc_htcp.c (revision 0d66206fff44f864ea8a4b220c3a53b4caa959a0)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
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/cdefs.h>
53 __FBSDID("$FreeBSD$");
54 
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>
64 
65 #include <net/vnet.h>
66 
67 #include <net/route.h>
68 #include <net/route/nhop.h>
69 
70 #include <netinet/in_pcb.h>
71 #include <netinet/tcp.h>
72 #include <netinet/tcp_seq.h>
73 #include <netinet/tcp_timer.h>
74 #include <netinet/tcp_var.h>
75 #include <netinet/cc/cc.h>
76 #include <netinet/cc/cc_module.h>
77 
78 /* Fixed point math shifts. */
79 #define HTCP_SHIFT 8
80 #define HTCP_ALPHA_INC_SHIFT 4
81 
82 #define HTCP_INIT_ALPHA 1
83 #define HTCP_DELTA_L hz		/* 1 sec in ticks. */
84 #define HTCP_MINBETA 128	/* 0.5 << HTCP_SHIFT. */
85 #define HTCP_MAXBETA 204	/* ~0.8 << HTCP_SHIFT. */
86 #define HTCP_MINROWE 26		/* ~0.1 << HTCP_SHIFT. */
87 #define HTCP_MAXROWE 512	/* 2 << HTCP_SHIFT. */
88 
89 /* RTT_ref (ms) used in the calculation of alpha if RTT scaling is enabled. */
90 #define HTCP_RTT_REF 100
91 
92 /* Don't trust SRTT until this many samples have been taken. */
93 #define HTCP_MIN_RTT_SAMPLES 8
94 
95 /*
96  * HTCP_CALC_ALPHA performs a fixed point math calculation to determine the
97  * value of alpha, based on the function defined in the HTCP spec.
98  *
99  * i.e. 1 + 10(delta - delta_l) + ((delta - delta_l) / 2) ^ 2
100  *
101  * "diff" is passed in to the macro as "delta - delta_l" and is expected to be
102  * in units of ticks.
103  *
104  * The joyousnous of fixed point maths means our function implementation looks a
105  * little funky...
106  *
107  * In order to maintain some precision in the calculations, a fixed point shift
108  * HTCP_ALPHA_INC_SHIFT is used to ensure the integer divisions don't
109  * truncate the results too badly.
110  *
111  * The "16" value is the "1" term in the alpha function shifted up by
112  * HTCP_ALPHA_INC_SHIFT
113  *
114  * The "160" value is the "10" multiplier in the alpha function multiplied by
115  * 2^HTCP_ALPHA_INC_SHIFT
116  *
117  * Specifying these as constants reduces the computations required. After
118  * up-shifting all the terms in the function and performing the required
119  * calculations, we down-shift the final result by HTCP_ALPHA_INC_SHIFT to
120  * ensure it is back in the correct range.
121  *
122  * The "hz" terms are required as kernels can be configured to run with
123  * different tick timers, which we have to adjust for in the alpha calculation
124  * (which originally was defined in terms of seconds).
125  *
126  * We also have to be careful to constrain the value of diff such that it won't
127  * overflow whilst performing the calculation. The middle term i.e. (160 * diff)
128  * / hz is the limiting factor in the calculation. We must constrain diff to be
129  * less than the max size of an int divided by the constant 160 figure
130  * i.e. diff < INT_MAX / 160
131  *
132  * NB: Changing HTCP_ALPHA_INC_SHIFT will require you to MANUALLY update the
133  * constants used in this function!
134  */
135 #define HTCP_CALC_ALPHA(diff) \
136 ((\
137 	(16) + \
138 	((160 * (diff)) / hz) + \
139 	(((diff) / hz) * (((diff) << HTCP_ALPHA_INC_SHIFT) / (4 * hz))) \
140 ) >> HTCP_ALPHA_INC_SHIFT)
141 
142 static void	htcp_ack_received(struct cc_var *ccv, uint16_t type);
143 static void	htcp_cb_destroy(struct cc_var *ccv);
144 static int	htcp_cb_init(struct cc_var *ccv, void *ptr);
145 static void	htcp_cong_signal(struct cc_var *ccv, uint32_t type);
146 static int	htcp_mod_init(void);
147 static void	htcp_post_recovery(struct cc_var *ccv);
148 static void	htcp_recalc_alpha(struct cc_var *ccv);
149 static void	htcp_recalc_beta(struct cc_var *ccv);
150 static void	htcp_record_rtt(struct cc_var *ccv);
151 static void	htcp_ssthresh_update(struct cc_var *ccv);
152 static size_t	htcp_data_sz(void);
153 
154 struct htcp {
155 	/* cwnd before entering cong recovery. */
156 	unsigned long	prev_cwnd;
157 	/* cwnd additive increase parameter. */
158 	int		alpha;
159 	/* cwnd multiplicative decrease parameter. */
160 	int		beta;
161 	/* Largest rtt seen for the flow. */
162 	int		maxrtt;
163 	/* Shortest rtt seen for the flow. */
164 	int		minrtt;
165 	/* Time of last congestion event in ticks. */
166 	int		t_last_cong;
167 };
168 
169 static int htcp_rtt_ref;
170 /*
171  * The maximum number of ticks the value of diff can reach in
172  * htcp_recalc_alpha() before alpha will stop increasing due to overflow.
173  * See comment above HTCP_CALC_ALPHA for more info.
174  */
175 static int htcp_max_diff = INT_MAX / ((1 << HTCP_ALPHA_INC_SHIFT) * 10);
176 
177 /* Per-netstack vars. */
178 VNET_DEFINE_STATIC(u_int, htcp_adaptive_backoff) = 0;
179 VNET_DEFINE_STATIC(u_int, htcp_rtt_scaling) = 0;
180 #define	V_htcp_adaptive_backoff    VNET(htcp_adaptive_backoff)
181 #define	V_htcp_rtt_scaling    VNET(htcp_rtt_scaling)
182 
183 struct cc_algo htcp_cc_algo = {
184 	.name = "htcp",
185 	.ack_received = htcp_ack_received,
186 	.cb_destroy = htcp_cb_destroy,
187 	.cb_init = htcp_cb_init,
188 	.cong_signal = htcp_cong_signal,
189 	.mod_init = htcp_mod_init,
190 	.post_recovery = htcp_post_recovery,
191 	.cc_data_sz = htcp_data_sz,
192 	.after_idle = newreno_cc_after_idle,
193 };
194 
195 static void
196 htcp_ack_received(struct cc_var *ccv, uint16_t type)
197 {
198 	struct htcp *htcp_data;
199 
200 	htcp_data = ccv->cc_data;
201 	htcp_record_rtt(ccv);
202 
203 	/*
204 	 * Regular ACK and we're not in cong/fast recovery and we're cwnd
205 	 * limited and we're either not doing ABC or are slow starting or are
206 	 * doing ABC and we've sent a cwnd's worth of bytes.
207 	 */
208 	if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
209 	    (ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
210 	    CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
211 	    (V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
212 		htcp_recalc_beta(ccv);
213 		htcp_recalc_alpha(ccv);
214 		/*
215 		 * Use the logic in NewReno ack_received() for slow start and
216 		 * for the first HTCP_DELTA_L ticks after either the flow starts
217 		 * or a congestion event (when alpha equals 1).
218 		 */
219 		if (htcp_data->alpha == 1 ||
220 		    CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh))
221 			newreno_cc_ack_received(ccv, type);
222 		else {
223 			if (V_tcp_do_rfc3465) {
224 				/* Increment cwnd by alpha segments. */
225 				CCV(ccv, snd_cwnd) += htcp_data->alpha *
226 				    CCV(ccv, t_maxseg);
227 				ccv->flags &= ~CCF_ABC_SENTAWND;
228 			} else
229 				/*
230 				 * Increment cwnd by alpha/cwnd segments to
231 				 * approximate an increase of alpha segments
232 				 * per RTT.
233 				 */
234 				CCV(ccv, snd_cwnd) += (((htcp_data->alpha <<
235 				    HTCP_SHIFT) / (CCV(ccv, snd_cwnd) /
236 				    CCV(ccv, t_maxseg))) * CCV(ccv, t_maxseg))
237 				    >> HTCP_SHIFT;
238 		}
239 	}
240 }
241 
242 static void
243 htcp_cb_destroy(struct cc_var *ccv)
244 {
245 	free(ccv->cc_data, M_CC_MEM);
246 }
247 
248 static size_t
249 htcp_data_sz(void)
250 {
251 	return(sizeof(struct htcp));
252 }
253 
254 static int
255 htcp_cb_init(struct cc_var *ccv, void *ptr)
256 {
257 	struct htcp *htcp_data;
258 
259 	INP_WLOCK_ASSERT(tptoinpcb(ccv->ccvc.tcp));
260 	if (ptr == NULL) {
261 		htcp_data = malloc(sizeof(struct htcp), M_CC_MEM, M_NOWAIT);
262 		if (htcp_data == NULL)
263 			return (ENOMEM);
264 	} else
265 		htcp_data = ptr;
266 
267 	/* Init some key variables with sensible defaults. */
268 	htcp_data->alpha = HTCP_INIT_ALPHA;
269 	htcp_data->beta = HTCP_MINBETA;
270 	htcp_data->maxrtt = TCPTV_SRTTBASE;
271 	htcp_data->minrtt = TCPTV_SRTTBASE;
272 	htcp_data->prev_cwnd = 0;
273 	htcp_data->t_last_cong = ticks;
274 
275 	ccv->cc_data = htcp_data;
276 
277 	return (0);
278 }
279 
280 /*
281  * Perform any necessary tasks before we enter congestion recovery.
282  */
283 static void
284 htcp_cong_signal(struct cc_var *ccv, uint32_t type)
285 {
286 	struct htcp *htcp_data;
287 	u_int mss;
288 
289 	htcp_data = ccv->cc_data;
290 	mss = tcp_maxseg(ccv->ccvc.tcp);
291 
292 	switch (type) {
293 	case CC_NDUPACK:
294 		if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
295 			if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
296 				/*
297 				 * Apply hysteresis to maxrtt to ensure
298 				 * reductions in the RTT are reflected in our
299 				 * measurements.
300 				 */
301 				htcp_data->maxrtt = (htcp_data->minrtt +
302 				    (htcp_data->maxrtt - htcp_data->minrtt) *
303 				    95) / 100;
304 				htcp_ssthresh_update(ccv);
305 				htcp_data->t_last_cong = ticks;
306 				htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
307 			}
308 			ENTER_RECOVERY(CCV(ccv, t_flags));
309 		}
310 		break;
311 
312 	case CC_ECN:
313 		if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
314 			/*
315 			 * Apply hysteresis to maxrtt to ensure reductions in
316 			 * the RTT are reflected in our measurements.
317 			 */
318 			htcp_data->maxrtt = (htcp_data->minrtt + (htcp_data->maxrtt -
319 			    htcp_data->minrtt) * 95) / 100;
320 			htcp_ssthresh_update(ccv);
321 			CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
322 			htcp_data->t_last_cong = ticks;
323 			htcp_data->prev_cwnd = CCV(ccv, snd_cwnd);
324 			ENTER_CONGRECOVERY(CCV(ccv, t_flags));
325 		}
326 		break;
327 
328 	case CC_RTO:
329 		CCV(ccv, snd_ssthresh) = max(min(CCV(ccv, snd_wnd),
330 						 CCV(ccv, snd_cwnd)) / 2 / mss,
331 					     2) * mss;
332 		CCV(ccv, snd_cwnd) = mss;
333 		/*
334 		 * Grab the current time and record it so we know when the
335 		 * most recent congestion event was. Only record it when the
336 		 * timeout has fired more than once, as there is a reasonable
337 		 * chance the first one is a false alarm and may not indicate
338 		 * congestion.
339 		 */
340 		if (CCV(ccv, t_rxtshift) >= 2)
341 			htcp_data->t_last_cong = ticks;
342 		break;
343 	}
344 }
345 
346 static int
347 htcp_mod_init(void)
348 {
349 	/*
350 	 * HTCP_RTT_REF is defined in ms, and t_srtt in the tcpcb is stored in
351 	 * units of TCP_RTT_SCALE*hz. Scale HTCP_RTT_REF to be in the same units
352 	 * as t_srtt.
353 	 */
354 	htcp_rtt_ref = (HTCP_RTT_REF * TCP_RTT_SCALE * hz) / 1000;
355 	return (0);
356 }
357 
358 /*
359  * Perform any necessary tasks before we exit congestion recovery.
360  */
361 static void
362 htcp_post_recovery(struct cc_var *ccv)
363 {
364 	int pipe;
365 	struct htcp *htcp_data;
366 
367 	pipe = 0;
368 	htcp_data = ccv->cc_data;
369 
370 	if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
371 		/*
372 		 * If inflight data is less than ssthresh, set cwnd
373 		 * conservatively to avoid a burst of data, as suggested in the
374 		 * NewReno RFC. Otherwise, use the HTCP method.
375 		 *
376 		 * XXXLAS: Find a way to do this without needing curack
377 		 */
378 		if (V_tcp_do_newsack)
379 			pipe = tcp_compute_pipe(ccv->ccvc.tcp);
380 		else
381 			pipe = CCV(ccv, snd_max) - ccv->curack;
382 
383 		if (pipe < CCV(ccv, snd_ssthresh))
384 			/*
385 			 * Ensure that cwnd down not collape to 1 MSS under
386 			 * adverse conditions. Implements RFC6582
387 			 */
388 			CCV(ccv, snd_cwnd) = max(pipe, CCV(ccv, t_maxseg)) +
389 			    CCV(ccv, t_maxseg);
390 		else
391 			CCV(ccv, snd_cwnd) = max(1, ((htcp_data->beta *
392 			    htcp_data->prev_cwnd / CCV(ccv, t_maxseg))
393 			    >> HTCP_SHIFT)) * CCV(ccv, t_maxseg);
394 	}
395 }
396 
397 static void
398 htcp_recalc_alpha(struct cc_var *ccv)
399 {
400 	struct htcp *htcp_data;
401 	int alpha, diff, now;
402 
403 	htcp_data = ccv->cc_data;
404 	now = ticks;
405 
406 	/*
407 	 * If ticks has wrapped around (will happen approximately once every 49
408 	 * days on a machine with the default kern.hz=1000) and a flow straddles
409 	 * the wrap point, our alpha calcs will be completely wrong. We cut our
410 	 * losses and restart alpha from scratch by setting t_last_cong = now -
411 	 * HTCP_DELTA_L.
412 	 *
413 	 * This does not deflate our cwnd at all. It simply slows the rate cwnd
414 	 * is growing by until alpha regains the value it held prior to taking
415 	 * this drastic measure.
416 	 */
417 	if (now < htcp_data->t_last_cong)
418 		htcp_data->t_last_cong = now - HTCP_DELTA_L;
419 
420 	diff = now - htcp_data->t_last_cong - HTCP_DELTA_L;
421 
422 	/* Cap alpha if the value of diff would overflow HTCP_CALC_ALPHA(). */
423 	if (diff < htcp_max_diff) {
424 		/*
425 		 * If it has been more than HTCP_DELTA_L ticks since congestion,
426 		 * increase alpha according to the function defined in the spec.
427 		 */
428 		if (diff > 0) {
429 			alpha = HTCP_CALC_ALPHA(diff);
430 
431 			/*
432 			 * Adaptive backoff fairness adjustment:
433 			 * 2 * (1 - beta) * alpha_raw
434 			 */
435 			if (V_htcp_adaptive_backoff)
436 				alpha = max(1, (2 * ((1 << HTCP_SHIFT) -
437 				    htcp_data->beta) * alpha) >> HTCP_SHIFT);
438 
439 			/*
440 			 * RTT scaling: (RTT / RTT_ref) * alpha
441 			 * alpha will be the raw value from HTCP_CALC_ALPHA() if
442 			 * adaptive backoff is off, or the adjusted value if
443 			 * adaptive backoff is on.
444 			 */
445 			if (V_htcp_rtt_scaling)
446 				alpha = max(1, (min(max(HTCP_MINROWE,
447 				    (CCV(ccv, t_srtt) << HTCP_SHIFT) /
448 				    htcp_rtt_ref), HTCP_MAXROWE) * alpha)
449 				    >> HTCP_SHIFT);
450 
451 		} else
452 			alpha = 1;
453 
454 		htcp_data->alpha = alpha;
455 	}
456 }
457 
458 static void
459 htcp_recalc_beta(struct cc_var *ccv)
460 {
461 	struct htcp *htcp_data;
462 
463 	htcp_data = ccv->cc_data;
464 
465 	/*
466 	 * TCPTV_SRTTBASE is the initialised value of each connection's SRTT, so
467 	 * we only calc beta if the connection's SRTT has been changed from its
468 	 * initial value. beta is bounded to ensure it is always between
469 	 * HTCP_MINBETA and HTCP_MAXBETA.
470 	 */
471 	if (V_htcp_adaptive_backoff && htcp_data->minrtt != TCPTV_SRTTBASE &&
472 	    htcp_data->maxrtt != TCPTV_SRTTBASE)
473 		htcp_data->beta = min(max(HTCP_MINBETA,
474 		    (htcp_data->minrtt << HTCP_SHIFT) / htcp_data->maxrtt),
475 		    HTCP_MAXBETA);
476 	else
477 		htcp_data->beta = HTCP_MINBETA;
478 }
479 
480 /*
481  * Record the minimum and maximum RTT seen for the connection. These are used in
482  * the calculation of beta if adaptive backoff is enabled.
483  */
484 static void
485 htcp_record_rtt(struct cc_var *ccv)
486 {
487 	struct htcp *htcp_data;
488 
489 	htcp_data = ccv->cc_data;
490 
491 	/* XXXLAS: Should there be some hysteresis for minrtt? */
492 
493 	/*
494 	 * Record the current SRTT as our minrtt if it's the smallest we've seen
495 	 * or minrtt is currently equal to its initialised value. Ignore SRTT
496 	 * until a min number of samples have been taken.
497 	 */
498 	if ((CCV(ccv, t_srtt) < htcp_data->minrtt ||
499 	    htcp_data->minrtt == TCPTV_SRTTBASE) &&
500 	    (CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES))
501 		htcp_data->minrtt = CCV(ccv, t_srtt);
502 
503 	/*
504 	 * Record the current SRTT as our maxrtt if it's the largest we've
505 	 * seen. Ignore SRTT until a min number of samples have been taken.
506 	 */
507 	if (CCV(ccv, t_srtt) > htcp_data->maxrtt
508 	    && CCV(ccv, t_rttupdated) >= HTCP_MIN_RTT_SAMPLES)
509 		htcp_data->maxrtt = CCV(ccv, t_srtt);
510 }
511 
512 /*
513  * Update the ssthresh in the event of congestion.
514  */
515 static void
516 htcp_ssthresh_update(struct cc_var *ccv)
517 {
518 	struct htcp *htcp_data;
519 
520 	htcp_data = ccv->cc_data;
521 
522 	/*
523 	 * On the first congestion event, set ssthresh to cwnd * 0.5, on
524 	 * subsequent congestion events, set it to cwnd * beta.
525 	 */
526 	if (CCV(ccv, snd_ssthresh) == TCP_MAXWIN << TCP_MAX_WINSHIFT)
527 		CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
528 		    HTCP_MINBETA) >> HTCP_SHIFT;
529 	else {
530 		htcp_recalc_beta(ccv);
531 		CCV(ccv, snd_ssthresh) = ((u_long)CCV(ccv, snd_cwnd) *
532 		    htcp_data->beta) >> HTCP_SHIFT;
533 	}
534 }
535 
536 SYSCTL_DECL(_net_inet_tcp_cc_htcp);
537 SYSCTL_NODE(_net_inet_tcp_cc, OID_AUTO, htcp, CTLFLAG_RW | CTLFLAG_MPSAFE, NULL,
538     "H-TCP related settings");
539 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, adaptive_backoff,
540     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_adaptive_backoff), 0,
541     "enable H-TCP adaptive backoff");
542 SYSCTL_UINT(_net_inet_tcp_cc_htcp, OID_AUTO, rtt_scaling,
543     CTLFLAG_VNET | CTLFLAG_RW, &VNET_NAME(htcp_rtt_scaling), 0,
544     "enable H-TCP RTT scaling");
545 
546 DECLARE_CC_MODULE(htcp, &htcp_cc_algo);
547 MODULE_VERSION(htcp, 2);
548