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Import a clean-room implementation of the experimental CUBIC congestion control

Browse Source 
algorithm based on the Internet-Draft "draft-rhee-tcpm-cubic-02.txt". It is
implemented as a kernel module compatible with the recently committed modular
congestion control framework.

CUBIC was designed for provide increased throughput in fast and long-distance
networks. It attempts to maintain fairness when competing with legacy NewReno
TCP in lower speed scenarios where NewReno is able to operate adequately. The
paper "CUBIC: A New TCP-Friendly High-Speed TCP Variant" provides additional
detail.

In collaboration with:	David Hayes <dahayes at swin edu au> and
			Grenville Armitage <garmitage at swin edu au>
Sponsored by:	FreeBSD Foundation
Reviewed by:	rpaulo (older patch from a few weeks ago)
MFC after:	3 months
This commit is contained in:
Lawrence Stewart 2010-12-02 06:05:44 +00:00
parent 5a7bfc90a9
commit 67fef78ba4
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=216114
4 changed files with 635 additions and 1 deletions
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2
sys/modules/cc/Makefile
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@ -1,5 +1,5 @@
# $FreeBSD$
SUBDIR=
SUBDIR= cc_cubic
.include <bsd.subdir.mk>

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sys/modules/cc/cc_cubic/Makefile Normal file
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# $FreeBSD$
.include <bsd.own.mk>
.PATH: ${.CURDIR}/../../../netinet/cc
KMOD= cc_cubic
SRCS= cc_cubic.c
.include <bsd.kmod.mk>

396
sys/netinet/cc/cc_cubic.c Normal file
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/*-
* Copyright (c) 2008-2010 Lawrence Stewart <lstewart@freebsd.org>
* Copyright (c) 2010 The FreeBSD Foundation
* All rights reserved.
*
* This software was developed by Lawrence Stewart while studying at the Centre
* for Advanced Internet Architectures, Swinburne University, made possible in
* part by a grant from the Cisco University Research Program Fund at Community
* Foundation Silicon Valley.
*
* Portions of this software were developed at the Centre for Advanced
* Internet Architectures, Swinburne University of Technology, Melbourne,
* Australia by David Hayes under sponsorship from the FreeBSD Foundation.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
/*
* An implementation of the CUBIC congestion control algorithm for FreeBSD,
* based on the Internet Draft "draft-rhee-tcpm-cubic-02" by Rhee, Xu and Ha.
* Originally released as part of the NewTCP research project at Swinburne
* University's Centre for Advanced Internet Architectures, Melbourne,
* Australia, which was made possible in part by a grant from the Cisco
* University Research Program Fund at Community Foundation Silicon Valley. More
* details are available at:
* http://caia.swin.edu.au/urp/newtcp/
*/
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/malloc.h>
#include <sys/module.h>
#include <sys/socket.h>
#include <sys/socketvar.h>
#include <sys/sysctl.h>
#include <sys/systm.h>
#include <net/vnet.h>
#include <netinet/cc.h>
#include <netinet/tcp_seq.h>
#include <netinet/tcp_timer.h>
#include <netinet/tcp_var.h>
#include <netinet/cc/cc_cubic.h>
#include <netinet/cc/cc_module.h>
static void cubic_ack_received(struct cc_var *ccv, uint16_t type);
static void cubic_cb_destroy(struct cc_var *ccv);
static int cubic_cb_init(struct cc_var *ccv);
static void cubic_cong_signal(struct cc_var *ccv, uint32_t type);
static void cubic_conn_init(struct cc_var *ccv);
static int cubic_mod_init(void);
static void cubic_post_recovery(struct cc_var *ccv);
static void cubic_record_rtt(struct cc_var *ccv);
static void cubic_ssthresh_update(struct cc_var *ccv);
struct cubic {
/* Cubic K in fixed point form with CUBIC_SHIFT worth of precision. */
int64_t K;
/* Sum of RTT samples across an epoch in ticks. */
int64_t sum_rtt_ticks;
/* cwnd at the most recent congestion event. */
unsigned long max_cwnd;
/* cwnd at the previous congestion event. */
unsigned long prev_max_cwnd;
/* Number of congestion events. */
uint32_t num_cong_events;
/* Minimum observed rtt in ticks. */
int min_rtt_ticks;
/* Mean observed rtt between congestion epochs. */
int mean_rtt_ticks;
/* ACKs since last congestion event. */
int epoch_ack_count;
/* Time of last congestion event in ticks. */
int t_last_cong;
};
MALLOC_DECLARE(M_CUBIC);
MALLOC_DEFINE(M_CUBIC, "cubic data",
"Per connection data required for the CUBIC congestion control algorithm");
struct cc_algo cubic_cc_algo = {
.name = "cubic",
.ack_received = cubic_ack_received,
.cb_destroy = cubic_cb_destroy,
.cb_init = cubic_cb_init,
.cong_signal = cubic_cong_signal,
.conn_init = cubic_conn_init,
.mod_init = cubic_mod_init,
.post_recovery = cubic_post_recovery,
};
static void
cubic_ack_received(struct cc_var *ccv, uint16_t type)
{
struct cubic *cubic_data;
unsigned long w_tf, w_cubic_next;
int ticks_since_cong;
cubic_data = ccv->cc_data;
cubic_record_rtt(ccv);
/*
* Regular ACK and we're not in cong/fast recovery and we're cwnd
* limited and we're either not doing ABC or are slow starting or are
* doing ABC and we've sent a cwnd's worth of bytes.
*/
if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
(ccv->flags & CCF_CWND_LIMITED) && (!V_tcp_do_rfc3465 ||
CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
(V_tcp_do_rfc3465 && ccv->flags & CCF_ABC_SENTAWND))) {
/* Use the logic in NewReno ack_received() for slow start. */
if (CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
cubic_data->min_rtt_ticks == TCPTV_SRTTBASE)
newreno_cc_algo.ack_received(ccv, type);
else {
ticks_since_cong = ticks - cubic_data->t_last_cong;
/*
* The mean RTT is used to best reflect the equations in
* the I-D. Using min_rtt in the tf_cwnd calculation
* causes w_tf to grow much faster than it should if the
* RTT is dominated by network buffering rather than
* propogation delay.
*/
w_tf = tf_cwnd(ticks_since_cong,
cubic_data->mean_rtt_ticks, cubic_data->max_cwnd,
CCV(ccv, t_maxseg));
w_cubic_next = cubic_cwnd(ticks_since_cong +
cubic_data->mean_rtt_ticks, cubic_data->max_cwnd,
CCV(ccv, t_maxseg), cubic_data->K);
ccv->flags &= ~CCF_ABC_SENTAWND;
if (w_cubic_next < w_tf)
/*
* TCP-friendly region, follow tf
* cwnd growth.
*/
CCV(ccv, snd_cwnd) = w_tf;
else if (CCV(ccv, snd_cwnd) < w_cubic_next) {
/*
* Concave or convex region, follow CUBIC
* cwnd growth.
*/
if (V_tcp_do_rfc3465)
CCV(ccv, snd_cwnd) = w_cubic_next;
else
CCV(ccv, snd_cwnd) += ((w_cubic_next -
CCV(ccv, snd_cwnd)) *
CCV(ccv, t_maxseg)) /
CCV(ccv, snd_cwnd);
}
/*
* If we're not in slow start and we're probing for a
* new cwnd limit at the start of a connection
* (happens when hostcache has a relevant entry),
* keep updating our current estimate of the
* max_cwnd.
*/
if (cubic_data->num_cong_events == 0 &&
cubic_data->max_cwnd < CCV(ccv, snd_cwnd))
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
}
}
}
static void
cubic_cb_destroy(struct cc_var *ccv)
{
if (ccv->cc_data != NULL)
free(ccv->cc_data, M_CUBIC);
}
static int
cubic_cb_init(struct cc_var *ccv)
{
struct cubic *cubic_data;
cubic_data = malloc(sizeof(struct cubic), M_CUBIC, M_NOWAIT|M_ZERO);
if (cubic_data == NULL)
return (ENOMEM);
/* Init some key variables with sensible defaults. */
cubic_data->t_last_cong = ticks;
cubic_data->min_rtt_ticks = TCPTV_SRTTBASE;
cubic_data->mean_rtt_ticks = TCPTV_SRTTBASE;
ccv->cc_data = cubic_data;
return (0);
}
/*
* Perform any necessary tasks before we enter congestion recovery.
*/
static void
cubic_cong_signal(struct cc_var *ccv, uint32_t type)
{
struct cubic *cubic_data;
cubic_data = ccv->cc_data;
switch (type) {
case CC_NDUPACK:
if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
cubic_ssthresh_update(ccv);
cubic_data->num_cong_events++;
cubic_data->prev_max_cwnd = cubic_data->max_cwnd;
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
}
ENTER_RECOVERY(CCV(ccv, t_flags));
}
break;
case CC_ECN:
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
cubic_ssthresh_update(ccv);
cubic_data->num_cong_events++;
cubic_data->prev_max_cwnd = cubic_data->max_cwnd;
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
cubic_data->t_last_cong = ticks;
CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
ENTER_CONGRECOVERY(CCV(ccv, t_flags));
}
break;
case CC_RTO:
/*
* Grab the current time and record it so we know when the
* most recent congestion event was. Only record it when the
* timeout has fired more than once, as there is a reasonable
* chance the first one is a false alarm and may not indicate
* congestion.
*/
if (CCV(ccv, t_rxtshift) >= 2)
cubic_data->num_cong_events++;
cubic_data->t_last_cong = ticks;
break;
}
}
static void
cubic_conn_init(struct cc_var *ccv)
{
struct cubic *cubic_data;
cubic_data = ccv->cc_data;
/*
* Ensure we have a sane initial value for max_cwnd recorded. Without
* this here bad things happen when entries from the TCP hostcache
* get used.
*/
cubic_data->max_cwnd = CCV(ccv, snd_cwnd);
}
static int
cubic_mod_init(void)
{
cubic_cc_algo.after_idle = newreno_cc_algo.after_idle;
return (0);
}
/*
* Perform any necessary tasks before we exit congestion recovery.
*/
static void
cubic_post_recovery(struct cc_var *ccv)
{
struct cubic *cubic_data;
cubic_data = ccv->cc_data;
/* Fast convergence heuristic. */
if (cubic_data->max_cwnd < cubic_data->prev_max_cwnd)
cubic_data->max_cwnd = (cubic_data->max_cwnd * CUBIC_FC_FACTOR)
>> CUBIC_SHIFT;
if (IN_FASTRECOVERY(CCV(ccv, t_flags))) {
/*
* If inflight data is less than ssthresh, set cwnd
* conservatively to avoid a burst of data, as suggested in
* the NewReno RFC. Otherwise, use the CUBIC method.
*
* XXXLAS: Find a way to do this without needing curack
*/
if (SEQ_GT(ccv->curack + CCV(ccv, snd_ssthresh),
CCV(ccv, snd_max)))
CCV(ccv, snd_cwnd) = CCV(ccv, snd_max) - ccv->curack +
CCV(ccv, t_maxseg);
else
/* Update cwnd based on beta and adjusted max_cwnd. */
CCV(ccv, snd_cwnd) = max(1, ((CUBIC_BETA *
cubic_data->max_cwnd) >> CUBIC_SHIFT));
}
cubic_data->t_last_cong = ticks;
/* Calculate the average RTT between congestion epochs. */
if (cubic_data->epoch_ack_count > 0 && cubic_data->sum_rtt_ticks > 0)
cubic_data->mean_rtt_ticks = (int)(cubic_data->sum_rtt_ticks /
cubic_data->epoch_ack_count);
else
/* For safety. */
cubic_data->mean_rtt_ticks = cubic_data->min_rtt_ticks;
cubic_data->epoch_ack_count = 0;
cubic_data->sum_rtt_ticks = 0;
cubic_data->K = cubic_k(cubic_data->max_cwnd / CCV(ccv, t_maxseg));
}
/*
* Record the min RTT and sum samples for the epoch average RTT calculation.
*/
static void
cubic_record_rtt(struct cc_var *ccv)
{
struct cubic *cubic_data;
int t_srtt_ticks;
/* Ignore srtt until a min number of samples have been taken. */
if (CCV(ccv, t_rttupdated) >= CUBIC_MIN_RTT_SAMPLES) {
cubic_data = ccv->cc_data;
t_srtt_ticks = CCV(ccv, t_srtt) / TCP_RTT_SCALE;
/*
* Record the current SRTT as our minrtt if it's the smallest
* we've seen or minrtt is currently equal to its initialised
* value.
*
* XXXLAS: Should there be some hysteresis for minrtt?
*/
if ((t_srtt_ticks < cubic_data->min_rtt_ticks ||
cubic_data->min_rtt_ticks == TCPTV_SRTTBASE))
cubic_data->min_rtt_ticks = max(1, t_srtt_ticks);
/* Sum samples for epoch average RTT calculation. */
cubic_data->sum_rtt_ticks += t_srtt_ticks;
cubic_data->epoch_ack_count++;
}
}
/*
* Update the ssthresh in the event of congestion.
*/
static void
cubic_ssthresh_update(struct cc_var *ccv)
{
struct cubic *cubic_data;
cubic_data = ccv->cc_data;
/*
* On the first congestion event, set ssthresh to cwnd * 0.5, on
* subsequent congestion events, set it to cwnd * beta.
*/
if (cubic_data->num_cong_events == 0)
CCV(ccv, snd_ssthresh) = CCV(ccv, snd_cwnd) >> 1;
else
CCV(ccv, snd_ssthresh) = (CCV(ccv, snd_cwnd) * CUBIC_BETA)
>> CUBIC_SHIFT;
}
DECLARE_CC_MODULE(cubic, &cubic_cc_algo);

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/*-
* Copyright (c) 2008-2010 Lawrence Stewart <lstewart@freebsd.org>
* Copyright (c) 2010 The FreeBSD Foundation
* All rights reserved.
*
* This software was developed by Lawrence Stewart while studying at the Centre
* for Advanced Internet Architectures, Swinburne University, made possible in
* part by a grant from the Cisco University Research Program Fund at Community
* Foundation Silicon Valley.
*
* Portions of this software were developed at the Centre for Advanced
* Internet Architectures, Swinburne University of Technology, Melbourne,
* Australia by David Hayes under sponsorship from the FreeBSD Foundation.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* $FreeBSD$
*/
#ifndef _NETINET_CC_CUBIC_H_
#define _NETINET_CC_CUBIC_H_
/* Number of bits of precision for fixed point math calcs. */
#define CUBIC_SHIFT 8
#define CUBIC_SHIFT_4 32
/* 0.5 << CUBIC_SHIFT. */
#define RENO_BETA 128
/* ~0.8 << CUBIC_SHIFT. */
#define CUBIC_BETA 204
/* ~0.2 << CUBIC_SHIFT. */
#define ONE_SUB_CUBIC_BETA 51
/* 3 * ONE_SUB_CUBIC_BETA. */
#define THREE_X_PT2 153
/* (2 << CUBIC_SHIFT) - ONE_SUB_CUBIC_BETA. */
#define TWO_SUB_PT2 461
/* ~0.4 << CUBIC_SHIFT. */
#define CUBIC_C_FACTOR 102
/* CUBIC fast convergence factor: ~0.9 << CUBIC_SHIFT. */
#define CUBIC_FC_FACTOR 230
/* Don't trust s_rtt until this many rtt samples have been taken. */
#define CUBIC_MIN_RTT_SAMPLES 8
/* Userland only bits. */
#ifndef _KERNEL
extern int hz;
/*
* Implementation based on the formulae found in the CUBIC Internet Draft
* "draft-rhee-tcpm-cubic-02".
*
* Note BETA used in cc_cubic is equal to (1-beta) in the I-D
*/
static __inline float
theoretical_cubic_k(double wmax_pkts)
{
double C;
C = 0.4;
return (pow((wmax_pkts * 0.2) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
}
static __inline unsigned long
theoretical_cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss)
{
double C, wmax_pkts;
C = 0.4;
wmax_pkts = wmax / (double)smss;
return (smss * (wmax_pkts +
(C * pow(ticks_since_cong / (double)hz -
theoretical_cubic_k(wmax_pkts) / pow(2, CUBIC_SHIFT), 3.0))));
}
static __inline unsigned long
theoretical_reno_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
uint32_t smss)
{
return ((wmax * 0.5) + ((ticks_since_cong / (float)rtt_ticks) * smss));
}
static __inline unsigned long
theoretical_tf_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
uint32_t smss)
{
return ((wmax * 0.8) + ((3 * 0.2) / (2 - 0.2) *
(ticks_since_cong / (float)rtt_ticks) * smss));
}
#endif /* !_KERNEL */
/*
* Compute the CUBIC K value used in the cwnd calculation, using an
* implementation of eqn 2 in the I-D. The method used
* here is adapted from Apple Computer Technical Report #KT-32.
*/
static __inline int64_t
cubic_k(unsigned long wmax_pkts)
{
int64_t s, K;
uint16_t p;
K = s = 0;
p = 0;
/* (wmax * beta)/C with CUBIC_SHIFT worth of precision. */
s = ((wmax_pkts * ONE_SUB_CUBIC_BETA) << CUBIC_SHIFT) / CUBIC_C_FACTOR;
/* Rebase s to be between 1 and 1/8 with a shift of CUBIC_SHIFT. */
while (s >= 256) {
s >>= 3;
p++;
}
/*
* Some magic constants taken from the Apple TR with appropriate
* shifts: 275 == 1.072302 << CUBIC_SHIFT, 98 == 0.3812513 <<
* CUBIC_SHIFT, 120 == 0.46946116 << CUBIC_SHIFT.
*/
K = (((s * 275) >> CUBIC_SHIFT) + 98) -
(((s * s * 120) >> CUBIC_SHIFT) >> CUBIC_SHIFT);
/* Multiply by 2^p to undo the rebasing of s from above. */
return (K <<= p);
}
/*
* Compute the new cwnd value using an implementation of eqn 1 from the I-D.
* Thanks to Kip Macy for help debugging this function.
*
* XXXLAS: Characterise bounds for overflow.
*/
static __inline unsigned long
cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss, int64_t K)
{
int64_t cwnd;
/* K is in fixed point form with CUBIC_SHIFT worth of precision. */
/* t - K, with CUBIC_SHIFT worth of precision. */
cwnd = ((int64_t)(ticks_since_cong << CUBIC_SHIFT) - (K * hz)) / hz;
/* (t - K)^3, with CUBIC_SHIFT^3 worth of precision. */
cwnd *= (cwnd * cwnd);
/*
* C(t - K)^3 + wmax
* The down shift by CUBIC_SHIFT_4 is because cwnd has 4 lots of
* CUBIC_SHIFT included in the value. 3 from the cubing of cwnd above,
* and an extra from multiplying through by CUBIC_C_FACTOR.
*/
cwnd = ((cwnd * CUBIC_C_FACTOR * smss) >> CUBIC_SHIFT_4) + wmax;
return ((unsigned long)cwnd);
}
/*
* Compute an approximation of the NewReno cwnd some number of ticks after a
* congestion event. RTT should be the average RTT estimate for the path
* measured over the previous congestion epoch and wmax is the value of cwnd at
* the last congestion event. The "TCP friendly" concept in the CUBIC I-D is
* rather tricky to understand and it turns out this function is not required.
* It is left here for reference.
*/
static __inline unsigned long
reno_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
uint32_t smss)
{
/*
* For NewReno, beta = 0.5, therefore: W_tcp(t) = wmax*0.5 + t/RTT
* W_tcp(t) deals with cwnd/wmax in pkts, so because our cwnd is in
* bytes, we have to multiply by smss.
*/
return (((wmax * RENO_BETA) + (((ticks_since_cong * smss)
<< CUBIC_SHIFT) / rtt_ticks)) >> CUBIC_SHIFT);
}
/*
* Compute an approximation of the "TCP friendly" cwnd some number of ticks
* after a congestion event that is designed to yield the same average cwnd as
* NewReno while using CUBIC's beta of 0.8. RTT should be the average RTT
* estimate for the path measured over the previous congestion epoch and wmax is
* the value of cwnd at the last congestion event.
*/
static __inline unsigned long
tf_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
uint32_t smss)
{
/* Equation 4 of I-D. */
return (((wmax * CUBIC_BETA) + (((THREE_X_PT2 * ticks_since_cong *
smss) << CUBIC_SHIFT) / TWO_SUB_PT2 / rtt_ticks)) >> CUBIC_SHIFT);
}
#endif /* _NETINET_CC_CUBIC_H_ */