git: a2f579635f67 - main - cc_cubic: sync to the new specification of RFC9438 for TCP CUBIC.
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Date: Mon, 16 Jun 2025 16:55:00 UTC
The branch main has been updated by cc:
URL: https://cgit.FreeBSD.org/src/commit/?id=a2f579635f6726b2eb3e7f622985fde1abcd0d82
commit a2f579635f6726b2eb3e7f622985fde1abcd0d82
Author: Cheng Cui <cc@FreeBSD.org>
AuthorDate: 2025-03-27 17:16:04 +0000
Commit: Cheng Cui <cc@FreeBSD.org>
CommitDate: 2025-06-16 16:52:46 +0000
cc_cubic: sync to the new specification of RFC9438 for TCP CUBIC.
Reviewed by: rscheff
Differential Revision: https://reviews.freebsd.org/D49540
---
sys/netinet/cc/cc_cubic.c | 164 ++++++++++++++++++++++++----------------------
sys/netinet/cc/cc_cubic.h | 160 ++++++++++++++++++++++++++------------------
2 files changed, 181 insertions(+), 143 deletions(-)
diff --git a/sys/netinet/cc/cc_cubic.c b/sys/netinet/cc/cc_cubic.c
index a2e72130fa88..b3e15009244d 100644
--- a/sys/netinet/cc/cc_cubic.c
+++ b/sys/netinet/cc/cc_cubic.c
@@ -38,7 +38,7 @@
/*
* 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.
+ * based on the Internet RFC9438 by Xu, Ha, Rhee, Goel, and Eggert.
* Originally released as part of the NewTCP research project at Swinburne
* University of Technology's Centre for Advanced Internet Architectures,
* Melbourne, Australia, which was made possible in part by a grant from the
@@ -81,7 +81,7 @@ 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, uint32_t maxseg);
+static uint32_t cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg);
static void cubic_after_idle(struct cc_var *ccv);
static size_t cubic_data_sz(void);
static void cubic_newround(struct cc_var *ccv, uint32_t round_cnt);
@@ -236,10 +236,11 @@ static void
cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
{
struct cubic *cubic_data;
- unsigned long W_est, W_cubic;
+ uint32_t W_est, W_cubic, cwin, target, incr;
int usecs_since_epoch;
uint32_t mss = tcp_fixed_maxseg(ccv->tp);
+ cwin = CCV(ccv, snd_cwnd);
cubic_data = ccv->cc_data;
cubic_record_rtt(ccv);
@@ -250,7 +251,7 @@ cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
(ccv->flags & CCF_CWND_LIMITED)) {
/* Use the logic in NewReno ack_received() for slow start. */
- if (CCV(ccv, snd_cwnd) <= CCV(ccv, snd_ssthresh) ||
+ if (cwin <= CCV(ccv, snd_ssthresh) ||
cubic_data->min_rtt_usecs == TCPTV_SRTTBASE) {
cubic_does_slow_start(ccv, cubic_data);
} else {
@@ -265,20 +266,32 @@ cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
cubic_data->flags &= ~CUBICFLAG_HYSTART_ENABLED;
cubic_log_hystart_event(ccv, cubic_data, 11, CCV(ccv, snd_ssthresh));
}
- if ((cubic_data->flags & CUBICFLAG_RTO_EVENT) &&
- (cubic_data->flags & CUBICFLAG_IN_SLOWSTART)) {
- /* RFC8312 Section 4.7 */
- cubic_data->flags &= ~(CUBICFLAG_RTO_EVENT |
- CUBICFLAG_IN_SLOWSTART);
- cubic_data->W_max = CCV(ccv, snd_cwnd);
- cubic_data->t_epoch = ticks;
- cubic_data->K = 0;
- } else if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
+ if (cubic_data->flags & (CUBICFLAG_IN_SLOWSTART |
+ CUBICFLAG_CONG_EVENT |
CUBICFLAG_IN_APPLIMIT)) {
+ /*
+ * At the beginning of the current congestion
+ * avoidance stage, The epoch variables
+ * (t_epoch, cwnd_epoch, K) are updated in the
+ * following three cases:
+ * 1) just exited the slow start
+ * 2) after a congestion event
+ * 3) application-limited
+ */
+ cubic_data->t_epoch = ticks;
+ cubic_data->cwnd_epoch = cwin;
+ cubic_data->K = cubic_k(cubic_data->W_max / mss,
+ cubic_data->cwnd_epoch / mss);
cubic_data->flags &= ~(CUBICFLAG_IN_SLOWSTART |
+ CUBICFLAG_CONG_EVENT |
CUBICFLAG_IN_APPLIMIT);
- cubic_data->t_epoch = ticks;
- cubic_data->K = cubic_k(cubic_data->W_max / mss);
+
+ if (cubic_data->flags & CUBICFLAG_RTO_EVENT) {
+ /* RFC9438 Section 4.8: Timeout */
+ cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
+ cubic_data->W_max = cwin;
+ cubic_data->K = 0;
+ }
}
usecs_since_epoch = (ticks - cubic_data->t_epoch) * tick;
if (usecs_since_epoch < 0) {
@@ -288,12 +301,9 @@ cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
usecs_since_epoch = INT_MAX;
cubic_data->t_epoch = ticks - INT_MAX;
}
-
W_est = tf_cwnd(ccv);
-
/*
- * The mean RTT is used to best reflect the equations in
- * the I-D.
+ * The mean RTT is used to best reflect the equations.
*/
W_cubic = cubic_cwnd(usecs_since_epoch +
cubic_data->mean_rtt_usecs,
@@ -302,33 +312,24 @@ cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
cubic_data->K);
if (W_cubic < W_est) {
- /*
- * TCP-friendly region, follow tf
- * cwnd growth.
- */
- CCV(ccv, snd_cwnd) = ulmin(W_est, INT_MAX);
+ /* RFC9438 Section 4.3: Reno-friendly region */
+ CCV(ccv, snd_cwnd) = W_est;
cubic_data->flags |= CUBICFLAG_IN_TF;
- } else if (CCV(ccv, snd_cwnd) < W_cubic) {
+ } else {
/*
- * Concave or convex region, follow CUBIC
- * cwnd growth.
- * Only update snd_cwnd, if it doesn't shrink.
+ * RFC9438 Section 4.4 or 4.5:
+ * Concave or Convex Region
*/
- CCV(ccv, snd_cwnd) = ulmin(W_cubic, INT_MAX);
- cubic_data->flags &= ~CUBICFLAG_IN_TF;
- }
-
- /*
- * 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
- * W_max.
- */
- if (((cubic_data->flags & CUBICFLAG_CONG_EVENT) == 0) &&
- cubic_data->W_max < CCV(ccv, snd_cwnd)) {
- cubic_data->W_max = CCV(ccv, snd_cwnd);
- cubic_data->K = cubic_k(cubic_data->W_max / mss);
+ if (W_cubic < cwin) {
+ target = cwin;
+ } else if (W_cubic > ((cwin * 3) >> 1)) {
+ target = (cwin * 3) >> 1;
+ } else {
+ target = W_cubic;
+ }
+ incr = (((target - cwin) << CUBIC_SHIFT) /
+ cwin * mss) >> CUBIC_SHIFT;
+ CCV(ccv, snd_cwnd) = cwin + incr;
}
}
} else if (type == CC_ACK && !IN_RECOVERY(CCV(ccv, t_flags)) &&
@@ -345,12 +346,11 @@ cubic_ack_received(struct cc_var *ccv, ccsignal_t type)
static void
cubic_after_idle(struct cc_var *ccv)
{
- struct cubic *cubic_data;
-
- cubic_data = ccv->cc_data;
+ struct cubic *cubic_data = ccv->cc_data;
+ uint32_t mss = tcp_fixed_maxseg(ccv->tp);
cubic_data->W_max = ulmax(cubic_data->W_max, CCV(ccv, snd_cwnd));
- cubic_data->K = cubic_k(cubic_data->W_max / tcp_fixed_maxseg(ccv->tp));
+ cubic_data->K = cubic_k(cubic_data->W_max / mss, cubic_data->cwnd_epoch / mss);
if ((cubic_data->flags & CUBICFLAG_HYSTART_ENABLED) == 0) {
/*
* Re-enable hystart if we have been idle.
@@ -389,7 +389,9 @@ cubic_cb_init(struct cc_var *ccv, void *ptr)
cubic_data = ptr;
/* Init some key variables with sensible defaults. */
- cubic_data->t_epoch = ticks;
+ cubic_data->t_epoch = 0;
+ cubic_data->cwnd_epoch = 0;
+ cubic_data->K = 0;
cubic_data->min_rtt_usecs = TCPTV_SRTTBASE;
cubic_data->mean_rtt_usecs = 1;
@@ -416,7 +418,7 @@ static void
cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
{
struct cubic *cubic_data;
- uint32_t mss, pipe;
+ uint32_t mss, pipe, ssthresh;
cubic_data = ccv->cc_data;
mss = tcp_fixed_maxseg(ccv->tp);
@@ -431,10 +433,13 @@ cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
}
if (!IN_FASTRECOVERY(CCV(ccv, t_flags))) {
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
- cubic_ssthresh_update(ccv, mss);
+ ssthresh = cubic_get_ssthresh(ccv, mss);
+ CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
+ /*
+ * The congestion flag will recalculate K at the
+ * beginning of the congestion avoidance stage.
+ */
cubic_data->flags |= CUBICFLAG_CONG_EVENT;
- cubic_data->t_epoch = ticks;
- cubic_data->K = cubic_k(cubic_data->W_max / mss);
}
ENTER_RECOVERY(CCV(ccv, t_flags));
}
@@ -448,17 +453,20 @@ cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
cubic_log_hystart_event(ccv, cubic_data, 9, CCV(ccv, snd_ssthresh));
}
if (!IN_CONGRECOVERY(CCV(ccv, t_flags))) {
- cubic_ssthresh_update(ccv, mss);
+ ssthresh = cubic_get_ssthresh(ccv, mss);
+ CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * mss);
+ CCV(ccv, snd_cwnd) = max(ssthresh, mss);
+ /*
+ * The congestion flag will recalculate K at the
+ * beginning of the congestion avoidance stage.
+ */
cubic_data->flags |= CUBICFLAG_CONG_EVENT;
- cubic_data->t_epoch = ticks;
- cubic_data->K = cubic_k(cubic_data->W_max / mss);
- CCV(ccv, snd_cwnd) = CCV(ccv, snd_ssthresh);
ENTER_CONGRECOVERY(CCV(ccv, t_flags));
}
break;
case CC_RTO:
- /* RFC8312 Section 4.7 */
+ /* RFC9438 Section 4.8: Timeout */
if (CCV(ccv, t_rxtshift) == 1) {
/*
* Remember the state only for the first RTO event. This
@@ -475,12 +483,16 @@ cubic_cong_signal(struct cc_var *ccv, ccsignal_t type)
(((uint64_t)min(CCV(ccv, snd_wnd), pipe) *
CUBIC_BETA) >> CUBIC_SHIFT) / mss) * mss;
}
- cubic_data->flags |= CUBICFLAG_CONG_EVENT | CUBICFLAG_RTO_EVENT;
+ /*
+ * The RTO flag will recalculate K at the
+ * beginning of the congestion avoidance stage.
+ */
+ cubic_data->flags |= CUBICFLAG_RTO_EVENT;
CCV(ccv, snd_cwnd) = mss;
break;
case CC_RTO_ERR:
- cubic_data->flags &= ~(CUBICFLAG_CONG_EVENT | CUBICFLAG_RTO_EVENT);
+ cubic_data->flags &= ~CUBICFLAG_RTO_EVENT;
cubic_data->K = cubic_data->undo_K;
cubic_data->W_max = cubic_data->undo_W_max;
cubic_data->cwnd_epoch = cubic_data->undo_cwnd_epoch;
@@ -503,7 +515,7 @@ cubic_conn_init(struct cc_var *ccv)
* this here bad things happen when entries from the TCP hostcache
* get used.
*/
- cubic_data->W_max = CCV(ccv, snd_cwnd);
+ cubic_data->W_max = UINT_MAX;
}
static int
@@ -603,44 +615,36 @@ cubic_record_rtt(struct cc_var *ccv)
}
/*
- * Update the ssthresh in the event of congestion.
+ * Return the new value for ssthresh in the event of a congestion.
*/
-static void
-cubic_ssthresh_update(struct cc_var *ccv, uint32_t maxseg)
+static uint32_t
+cubic_get_ssthresh(struct cc_var *ccv, uint32_t maxseg)
{
struct cubic *cubic_data;
- uint32_t ssthresh;
- uint32_t cwnd;
+ uint32_t cwnd, pipe;
cubic_data = ccv->cc_data;
cwnd = CCV(ccv, snd_cwnd);
- /* Fast convergence heuristic. */
+ /* RFC9438 Section 4.7: Fast convergence */
if (cwnd < cubic_data->W_max) {
cwnd = ((uint64_t)cwnd * CUBIC_FC_FACTOR) >> CUBIC_SHIFT;
}
- cubic_data->undo_W_max = cubic_data->W_max;
cubic_data->W_max = cwnd;
if (cubic_data->flags & CUBICFLAG_IN_TF) {
- /* If in the TCP friendly region, follow what newreno does */
- ssthresh = newreno_cc_cwnd_on_multiplicative_decrease(ccv, maxseg);
+ /* If in the TCP friendly region, follow what newreno does. */
+ return (newreno_cc_cwnd_on_multiplicative_decrease(ccv, maxseg));
- } else if ((cubic_data->flags & CUBICFLAG_CONG_EVENT) == 0) {
- /*
- * On the first congestion event, set ssthresh to cwnd * 0.5
- * and reduce W_max to cwnd * beta. This aligns the cubic
- * concave region appropriately.
- */
- ssthresh = cwnd >> 1;
- cubic_data->W_max = ((uint64_t)cwnd * CUBIC_BETA) >> CUBIC_SHIFT;
} else {
/*
- * On subsequent congestion events, set ssthresh to cwnd * beta.
+ * RFC9438 Section 4.6: Multiplicative Decrease
+ * Outside the TCP friendly region, set ssthresh to the size of
+ * inflight_size * beta.
*/
- ssthresh = ((uint64_t)cwnd * CUBIC_BETA) >> CUBIC_SHIFT;
+ pipe = tcp_compute_pipe(ccv->tp);
+ return ((pipe * CUBIC_BETA) >> CUBIC_SHIFT);
}
- CCV(ccv, snd_ssthresh) = max(ssthresh, 2 * maxseg);
}
static void
diff --git a/sys/netinet/cc/cc_cubic.h b/sys/netinet/cc/cc_cubic.h
index c30128570ab0..c31506d26b00 100644
--- a/sys/netinet/cc/cc_cubic.h
+++ b/sys/netinet/cc/cc_cubic.h
@@ -88,14 +88,23 @@
/* Kernel only bits */
#ifdef _KERNEL
struct cubic {
- /* CUBIC K in fixed point form with CUBIC_SHIFT worth of precision. */
+ /*
+ * CUBIC K in fixed point form with CUBIC_SHIFT worth of precision.
+ * Also means the time period in seconds it takes to increase the
+ * congestion window size at the beginning of the current congestion
+ * avoidance stage to W_max.
+ */
int64_t K;
/* Sum of RTT samples across an epoch in usecs. */
int64_t sum_rtt_usecs;
- /* Size of cwnd just before cwnd was reduced in the last congestion event */
- uint64_t W_max;
- /* The cwnd at the beginning of the current congestion avoidance stage */
- uint64_t cwnd_epoch;
+ /* Size of cwnd (in bytes) just before cwnd was reduced in the last congestion event. */
+ uint32_t W_max;
+ /* An estimate (in bytes) for the congestion window in the Reno-friendly region */
+ uint32_t W_est;
+ /* An estimate (in bytes) for the congestion window in the CUBIC region */
+ uint32_t W_cubic;
+ /* The cwnd (in bytes) at the beginning of the current congestion avoidance stage. */
+ uint32_t cwnd_epoch;
/* various flags */
uint32_t flags;
/* Minimum observed rtt in usecs. */
@@ -110,8 +119,8 @@ struct cubic {
int undo_t_epoch;
/* Few variables to restore the state after RTO_ERR */
int64_t undo_K;
- uint64_t undo_W_max;
- uint64_t undo_cwnd_epoch;
+ uint32_t undo_W_max;
+ uint32_t undo_cwnd_epoch;
uint32_t css_baseline_minrtt;
uint32_t css_current_round_minrtt;
uint32_t css_lastround_minrtt;
@@ -130,60 +139,103 @@ struct cubic {
extern int hz;
/*
- * Implementation based on the formulae found in the CUBIC Internet Draft
- * "draft-ietf-tcpm-cubic-04".
+ * Implementation based on the formulas in RFC9438.
*
*/
-static __inline float
-theoretical_cubic_k(double wmax_pkts)
+
+/*
+ * Returns K, the time period in seconds it takes to increase the congestion
+ * window size at the beginning of the current congestion avoidance stage to
+ * W_max.
+ */
+static inline float
+theoretical_cubic_k(uint32_t wmax_segs, uint32_t cwnd_epoch_segs)
{
double C;
C = 0.4;
+ if (wmax_segs <= cwnd_epoch_segs)
+ return 0.0;
- return (pow((wmax_pkts * 0.3) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
+ /*
+ * Figure 2: K = ((W_max - cwnd_epoch) / C)^(1/3)
+ */
+ return (pow((wmax_segs - cwnd_epoch_segs) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
}
-static __inline unsigned long
-theoretical_cubic_cwnd(int ticks_since_epoch, unsigned long wmax, uint32_t smss)
+/*
+ * Returns the congestion window in segments at time t in seconds based on the
+ * cubic increase function, where t is the elapsed time in seconds from the
+ * beginning of the current congestion avoidance stage, as described in RFC9438
+ * Section 4.2.
+ */
+static inline unsigned long
+theoretical_cubic_cwnd(int ticks_elapsed, uint32_t wmax_segs, uint32_t cwnd_epoch_segs)
{
- double C, wmax_pkts;
+ double C, t;
+ float K;
C = 0.4;
- wmax_pkts = wmax / (double)smss;
+ t = ticks_elapsed / (double)hz;
+ K = theoretical_cubic_k(wmax_segs, cwnd_epoch_segs);
- return (smss * (wmax_pkts +
- (C * pow(ticks_since_epoch / (double)hz -
- theoretical_cubic_k(wmax_pkts) / pow(2, CUBIC_SHIFT), 3.0))));
+ /*
+ * Figure 1: W_cubic(t) = C * (t - K)^3 + W_max
+ */
+ return (C * pow(t - K / pow(2, CUBIC_SHIFT), 3.0) + wmax_segs);
}
-static __inline unsigned long
-theoretical_reno_cwnd(int ticks_since_epoch, int rtt_ticks, unsigned long wmax,
- uint32_t smss)
+/*
+ * Returns estimated Reno congestion window in segments.
+ */
+static inline unsigned long
+theoretical_reno_cwnd(int ticks_elapsed, int rtt_ticks, uint32_t wmax_segs)
{
- return ((wmax * 0.5) + ((ticks_since_epoch / (float)rtt_ticks) * smss));
+ return (wmax_segs * 0.5 + ticks_elapsed / (float)rtt_ticks);
}
-static __inline unsigned long
-theoretical_tf_cwnd(int ticks_since_epoch, int rtt_ticks, unsigned long wmax,
- uint32_t smss)
+/*
+ * Returns an estimate for the congestion window in segments in the
+ * Reno-friendly region -- that is, an estimate for the congestion window of
+ * Reno, as described in RFC9438 Section 4.3, where:
+ * cwnd: Current congestion window in segments.
+ * cwnd_prior: Size of cwnd in segments at the time of setting ssthresh most
+ * recently, either upon exiting the first slow start or just before
+ * cwnd was reduced in the last congestion event.
+ * W_est: An estimate for the congestion window in segments in the Reno-friendly
+ * region -- that is, an estimate for the congestion window of Reno.
+ */
+static inline unsigned long
+theoretical_tf_cwnd(unsigned long W_est, unsigned long segs_acked, unsigned long cwnd,
+ unsigned long cwnd_prior)
{
+ float cubic_alpha, cubic_beta;
+
+ /* RFC9438 Section 4.6: The parameter β_cubic SHOULD be set to 0.7. */
+ cubic_beta = 0.7;
- return ((wmax * 0.7) + ((3 * 0.3) / (2 - 0.3) *
- (ticks_since_epoch / (float)rtt_ticks) * smss));
+ if (W_est >= cwnd_prior)
+ cubic_alpha = 1.0;
+ else
+ cubic_alpha = (3.0 * (1.0 - cubic_beta)) / (1.0 + cubic_beta);
+
+ /*
+ * Figure 4: W_est = W_est + α_cubic * segments_acked / cwnd
+ */
+ return (W_est + cubic_alpha * segs_acked / cwnd);
}
#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.
+ * implementation mentioned in Figure. 2 of RFC9438.
+ * The method used here is adapted from Apple Computer Technical Report #KT-32.
*/
-static __inline int64_t
-cubic_k(unsigned long wmax_pkts)
+static inline int64_t
+cubic_k(uint32_t wmax_segs, uint32_t cwnd_epoch_segs)
{
int64_t s, K;
uint16_t p;
@@ -191,8 +243,13 @@ cubic_k(unsigned long wmax_pkts)
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;
+ /* Handle the corner case where W_max <= cwnd_epoch */
+ if (wmax_segs <= cwnd_epoch_segs) {
+ return 0;
+ }
+
+ /* (wmax - cwnd_epoch) / C with CUBIC_SHIFT worth of precision. */
+ s = ((wmax_segs - cwnd_epoch_segs) << (2 * CUBIC_SHIFT)) / CUBIC_C_FACTOR;
/* Rebase s to be between 1 and 1/8 with a shift of CUBIC_SHIFT. */
while (s >= 256) {
@@ -213,13 +270,14 @@ cubic_k(unsigned long wmax_pkts)
}
/*
- * Compute the new cwnd value using an implementation of eqn 1 from the I-D.
+ * Compute and return the new cwnd value in bytes using an implementation
+ * mentioned in Figure. 1 of RFC9438.
* Thanks to Kip Macy for help debugging this function.
*
* XXXLAS: Characterise bounds for overflow.
*/
-static __inline unsigned long
-cubic_cwnd(int usecs_since_epoch, unsigned long wmax, uint32_t smss, int64_t K)
+static inline uint32_t
+cubic_cwnd(int usecs_since_epoch, uint32_t wmax, uint32_t smss, int64_t K)
{
int64_t cwnd;
@@ -238,7 +296,7 @@ cubic_cwnd(int usecs_since_epoch, unsigned long wmax, uint32_t smss, int64_t K)
cwnd *= (cwnd * cwnd);
/*
- * C(t - K)^3 + wmax
+ * Figure 1: 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.
@@ -252,34 +310,10 @@ cubic_cwnd(int usecs_since_epoch, unsigned long wmax, uint32_t smss, int64_t K)
return (lmax(0,cwnd));
}
-/*
- * Compute an approximation of the NewReno cwnd some number of usecs 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.
- *
- * XXX: Not used
- */
-static __inline unsigned long
-reno_cwnd(int usecs_since_epoch, int rtt_usecs, 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) + (((usecs_since_epoch * smss)
- << CUBIC_SHIFT) / rtt_usecs)) >> CUBIC_SHIFT);
-}
-
/*
* Compute the "TCP friendly" cwnd by newreno in congestion avoidance state.
*/
-static __inline unsigned long
+static inline uint32_t
tf_cwnd(struct cc_var *ccv)
{
/* newreno is "TCP friendly" */