cc_cubic.h

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/*-
 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
 *
 * 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 of Technology, 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_
 
#include <sys/limits.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
 
/* ~0.7 << CUBIC_SHIFT. */
#define CUBIC_BETA              179
 
/* ~0.3 << CUBIC_SHIFT. */
#define ONE_SUB_CUBIC_BETA      77
 
/* 3 * ONE_SUB_CUBIC_BETA. */
#define THREE_X_PT3             231
 
/* (2 << CUBIC_SHIFT) - ONE_SUB_CUBIC_BETA. */
#define TWO_SUB_PT3             435
 
/* ~0.4 << CUBIC_SHIFT. */
#define CUBIC_C_FACTOR          102
 
/* CUBIC fast convergence factor: (1+beta_cubic)/2. */
#define CUBIC_FC_FACTOR         217
 
/* Don't trust s_rtt until this many rtt samples have been taken. */
#define CUBIC_MIN_RTT_SAMPLES   8
 
/*
 * (2^21)^3 is long max. Dividing (2^63) by Cubic_C_factor
 * and taking cube-root yields 448845 as the effective useful limit
 */
#define CUBED_ROOT_MAX_ULONG    448845
 
/* Flags used in the cubic structure */
#define CUBICFLAG_CONG_EVENT            0x00000001      /* congestion experienced */
#define CUBICFLAG_IN_SLOWSTART          0x00000002      /* in slow start */
#define CUBICFLAG_IN_APPLIMIT           0x00000004      /* application limited */
#define CUBICFLAG_RTO_EVENT             0x00000008      /* RTO experienced */
#define CUBICFLAG_HYSTART_ENABLED       0x00000010      /* Hystart++ is enabled */
#define CUBICFLAG_HYSTART_IN_CSS        0x00000020      /* We are in Hystart++ CSS */
 
/* Kernel only bits */
#ifdef _KERNEL
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;
        /* A copy of prev_max_cwnd. Used for CC_RTO_ERR */
        unsigned long   prev_max_cwnd_cp;
        /* various flags */
        uint32_t        flags;
        /* 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;
        /* Timestamp (in ticks) of arriving in congestion avoidance from last
         * congestion event.
         */
        int             t_last_cong;
        /* Timestamp (in ticks) of a previous congestion event. Used for
         * CC_RTO_ERR.
         */
        int             t_last_cong_prev;
        uint32_t css_baseline_minrtt;
        uint32_t css_current_round_minrtt;
        uint32_t css_lastround_minrtt;
        uint32_t css_rttsample_count;
        uint32_t css_entered_at_round;
        uint32_t css_current_round;
        uint32_t css_fas_at_css_entry;
        uint32_t css_lowrtt_fas;
        uint32_t css_last_fas;
};
#endif
 
/* Userland only bits. */
#ifndef _KERNEL
 
extern int hz;
 
/*
 * Implementation based on the formulae found in the CUBIC Internet Draft
 * "draft-ietf-tcpm-cubic-04".
 *
 */
 
static __inline float
theoretical_cubic_k(double wmax_pkts)
{
        double C;
 
        C = 0.4;
 
        return (pow((wmax_pkts * 0.3) / 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 * 0.7) + ((3 * 0.3) / (2 - 0.3) *
            (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;
 
        if (cwnd > CUBED_ROOT_MAX_ULONG)
                return INT_MAX;
        if (cwnd < -CUBED_ROOT_MAX_ULONG)
                return 0;
 
        /* (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) >> CUBIC_SHIFT_4) * smss + wmax;
 
        /*
         * for negative cwnd, limiting to zero as lower bound
         */
        return (lmax(0,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 0.7. 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_PT3 * (unsigned long)ticks_since_cong *
            (unsigned long)smss) << CUBIC_SHIFT) / (TWO_SUB_PT3 * rtt_ticks)))
            >> CUBIC_SHIFT);
}
 
#endif /* _NETINET_CC_CUBIC_H_ */