FreeBSD/Linux Kernel Cross Reference
sys/netinet/cc/cc_cubic.h

     /*-
      * 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_ */

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