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/*
* Copyright (c) 2018 Cisco and/or its affiliates.
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <vnet/tcp/tcp.h>
#include <math.h>
#define beta_cubic 0.7
#define cubic_c 0.4
#define west_const (3 * (1 - beta_cubic) / (1 + beta_cubic))
typedef struct cubic_data_
{
/** time period (in seconds) needed to increase the current window
* size to W_max if there are no further congestion events */
f64 K;
/** time (in sec) since the start of current congestion avoidance */
f64 t_start;
/** Inflection point of the cubic function */
u32 w_max;
} __clib_packed cubic_data_t;
STATIC_ASSERT (sizeof (cubic_data_t) <= TCP_CC_DATA_SZ, "cubic data len");
static inline f64
cubic_time (u32 thread_index)
{
return transport_time_now (thread_index);
}
/**
* RFC 8312 Eq. 1
*
* CUBIC window increase function. Time and K need to be provided in seconds.
*/
static inline u64
W_cubic (cubic_data_t * cd, f64 t)
{
f64 diff = t - cd->K;
/* W_cubic(t) = C*(t-K)^3 + W_max */
return cubic_c * diff * diff * diff + cd->w_max;
}
/**
* RFC 8312 Eq. 2
*/
static inline f64
K_cubic (cubic_data_t * cd)
{
/* K = cubic_root(W_max*(1-beta_cubic)/C) */
return pow (cd->w_max * (1 - beta_cubic) / cubic_c, 1 / 3.0);
}
/**
* RFC 8312 Eq. 4
*
* Estimates the window size of AIMD(alpha_aimd, beta_aimd) for
* alpha_aimd=3*(1-beta_cubic)/(1+beta_cubic) and beta_aimd=beta_cubic.
* Time (t) and rtt should be provided in seconds
*/
static inline u32
W_est (cubic_data_t * cd, f64 t, f64 rtt)
{
/* W_est(t) = W_max*beta_cubic+[3*(1-beta_cubic)/(1+beta_cubic)]*(t/RTT) */
return cd->w_max * beta_cubic + west_const * (t / rtt);
}
static void
cubic_congestion (tcp_connection_t * tc)
{
cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
cd->w_max = tc->cwnd / tc->snd_mss;
tc->ssthresh = clib_max (tc->cwnd * beta_cubic, 2 * tc->snd_mss);
}
static void
cubic_recovered (tcp_connection_t * tc)
{
cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
cd->t_start = cubic_time (tc->c_thread_index);
cd->K = K_cubic (cd);
tc->cwnd = tc->ssthresh;
}
static void
cubic_rcv_ack (tcp_connection_t * tc)
{
cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
u64 w_cubic, w_aimd;
f64 t, rtt_sec;
u32 thresh;
/* Constrained by tx fifo, can't grow further */
if (tc->cwnd >= tc->tx_fifo_size)
return;
if (tcp_in_slowstart (tc))
{
tc->cwnd += clib_min (tc->snd_mss, tc->bytes_acked);
return;
}
t = cubic_time (tc->c_thread_index) - cd->t_start;
rtt_sec = clib_min (tc->mrtt_us, (f64) tc->srtt * TCP_TICK);
w_cubic = W_cubic (cd, t + rtt_sec) * tc->snd_mss;
w_aimd = W_est (cd, t, rtt_sec) * tc->snd_mss;
if (w_cubic < w_aimd)
{
tcp_cwnd_accumulate (tc, tc->cwnd, tc->bytes_acked);
}
else
{
if (w_cubic > tc->cwnd)
{
/* For NewReno and slow start, we increment cwnd based on the
* number of bytes acked, not the number of acks received. In
* particular, for NewReno we increment the cwnd by 1 snd_mss
* only after we accumulate 1 cwnd of acked bytes (RFC 3465).
*
* For Cubic, as per RFC 8312 we should increment cwnd by
* (w_cubic - cwnd)/cwnd for each ack. Instead of using that,
* we compute the number of packets that need to be acked
* before adding snd_mss to cwnd and compute the threshold
*/
thresh = (tc->snd_mss * tc->cwnd) / (w_cubic - tc->cwnd);
/* Make sure we don't increase cwnd more often than every
* 2 segments */
thresh = clib_max (thresh, 2 * tc->snd_mss);
}
else
{
/* Practically we can't increment so just inflate threshold */
thresh = 1000 * tc->cwnd;
}
tcp_cwnd_accumulate (tc, thresh, tc->bytes_acked);
}
}
static void
cubic_conn_init (tcp_connection_t * tc)
{
cubic_data_t *cd = (cubic_data_t *) tcp_cc_data (tc);
tc->ssthresh = tc->snd_wnd;
tc->cwnd = tcp_initial_cwnd (tc);
cd->w_max = 0;
cd->K = 0;
cd->t_start = cubic_time (tc->c_thread_index);
}
const static tcp_cc_algorithm_t tcp_cubic = {
.congestion = cubic_congestion,
.recovered = cubic_recovered,
.rcv_ack = cubic_rcv_ack,
.rcv_cong_ack = newreno_rcv_cong_ack,
.init = cubic_conn_init
};
clib_error_t *
cubic_init (vlib_main_t * vm)
{
clib_error_t *error = 0;
tcp_cc_algo_register (TCP_CC_CUBIC, &tcp_cubic);
return error;
}
VLIB_INIT_FUNCTION (cubic_init);
/*
* fd.io coding-style-patch-verification: ON
*
* Local Variables:
* eval: (c-set-style "gnu")
* End:
*/
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