The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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

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    1 /*-
    2  * Copyright (c) 2008-2010 Lawrence Stewart <lstewart@freebsd.org>
    3  * Copyright (c) 2010 The FreeBSD Foundation
    4  * All rights reserved.
    5  *
    6  * This software was developed by Lawrence Stewart while studying at the Centre
    7  * for Advanced Internet Architectures, Swinburne University of Technology, made
    8  * possible in part by a grant from the Cisco University Research Program Fund
    9  * at Community Foundation Silicon Valley.
   10  *
   11  * Portions of this software were developed at the Centre for Advanced
   12  * Internet Architectures, Swinburne University of Technology, Melbourne,
   13  * Australia by David Hayes under sponsorship from the FreeBSD Foundation.
   14  *
   15  * Redistribution and use in source and binary forms, with or without
   16  * modification, are permitted provided that the following conditions
   17  * are met:
   18  * 1. Redistributions of source code must retain the above copyright
   19  *    notice, this list of conditions and the following disclaimer.
   20  * 2. Redistributions in binary form must reproduce the above copyright
   21  *    notice, this list of conditions and the following disclaimer in the
   22  *    documentation and/or other materials provided with the distribution.
   23  *
   24  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   25  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   26  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   27  * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   28  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   29  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   30  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   31  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   32  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   33  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   34  * SUCH DAMAGE.
   35  *
   36  * $FreeBSD: releng/10.3/sys/netinet/cc/cc_cubic.h 220560 2011-04-12 08:13:18Z lstewart $
   37  */
   38 
   39 #ifndef _NETINET_CC_CUBIC_H_
   40 #define _NETINET_CC_CUBIC_H_
   41 
   42 /* Number of bits of precision for fixed point math calcs. */
   43 #define CUBIC_SHIFT             8
   44 
   45 #define CUBIC_SHIFT_4           32
   46 
   47 /* 0.5 << CUBIC_SHIFT. */
   48 #define RENO_BETA               128
   49 
   50 /* ~0.8 << CUBIC_SHIFT. */
   51 #define CUBIC_BETA              204
   52 
   53 /* ~0.2 << CUBIC_SHIFT. */
   54 #define ONE_SUB_CUBIC_BETA      51
   55 
   56 /* 3 * ONE_SUB_CUBIC_BETA. */
   57 #define THREE_X_PT2             153
   58 
   59 /* (2 << CUBIC_SHIFT) - ONE_SUB_CUBIC_BETA. */
   60 #define TWO_SUB_PT2             461
   61 
   62 /* ~0.4 << CUBIC_SHIFT. */
   63 #define CUBIC_C_FACTOR          102
   64 
   65 /* CUBIC fast convergence factor: ~0.9 << CUBIC_SHIFT. */
   66 #define CUBIC_FC_FACTOR         230
   67 
   68 /* Don't trust s_rtt until this many rtt samples have been taken. */
   69 #define CUBIC_MIN_RTT_SAMPLES   8
   70 
   71 /* Userland only bits. */
   72 #ifndef _KERNEL
   73 
   74 extern int hz;
   75 
   76 /*
   77  * Implementation based on the formulae found in the CUBIC Internet Draft
   78  * "draft-rhee-tcpm-cubic-02".
   79  *
   80  * Note BETA used in cc_cubic is equal to (1-beta) in the I-D
   81  */
   82 
   83 static __inline float
   84 theoretical_cubic_k(double wmax_pkts)
   85 {
   86         double C;
   87 
   88         C = 0.4;
   89 
   90         return (pow((wmax_pkts * 0.2) / C, (1.0 / 3.0)) * pow(2, CUBIC_SHIFT));
   91 }
   92 
   93 static __inline unsigned long
   94 theoretical_cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss)
   95 {
   96         double C, wmax_pkts;
   97 
   98         C = 0.4;
   99         wmax_pkts = wmax / (double)smss;
  100 
  101         return (smss * (wmax_pkts +
  102             (C * pow(ticks_since_cong / (double)hz -
  103             theoretical_cubic_k(wmax_pkts) / pow(2, CUBIC_SHIFT), 3.0))));
  104 }
  105 
  106 static __inline unsigned long
  107 theoretical_reno_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
  108     uint32_t smss)
  109 {
  110 
  111         return ((wmax * 0.5) + ((ticks_since_cong / (float)rtt_ticks) * smss));
  112 }
  113 
  114 static __inline unsigned long
  115 theoretical_tf_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
  116     uint32_t smss)
  117 {
  118 
  119         return ((wmax * 0.8) + ((3 * 0.2) / (2 - 0.2) *
  120             (ticks_since_cong / (float)rtt_ticks) * smss));
  121 }
  122 
  123 #endif /* !_KERNEL */
  124 
  125 /*
  126  * Compute the CUBIC K value used in the cwnd calculation, using an
  127  * implementation of eqn 2 in the I-D. The method used
  128  * here is adapted from Apple Computer Technical Report #KT-32.
  129  */
  130 static __inline int64_t
  131 cubic_k(unsigned long wmax_pkts)
  132 {
  133         int64_t s, K;
  134         uint16_t p;
  135 
  136         K = s = 0;
  137         p = 0;
  138 
  139         /* (wmax * beta)/C with CUBIC_SHIFT worth of precision. */
  140         s = ((wmax_pkts * ONE_SUB_CUBIC_BETA) << CUBIC_SHIFT) / CUBIC_C_FACTOR;
  141 
  142         /* Rebase s to be between 1 and 1/8 with a shift of CUBIC_SHIFT. */
  143         while (s >= 256) {
  144                 s >>= 3;
  145                 p++;
  146         }
  147 
  148         /*
  149          * Some magic constants taken from the Apple TR with appropriate
  150          * shifts: 275 == 1.072302 << CUBIC_SHIFT, 98 == 0.3812513 <<
  151          * CUBIC_SHIFT, 120 == 0.46946116 << CUBIC_SHIFT.
  152          */
  153         K = (((s * 275) >> CUBIC_SHIFT) + 98) -
  154             (((s * s * 120) >> CUBIC_SHIFT) >> CUBIC_SHIFT);
  155 
  156         /* Multiply by 2^p to undo the rebasing of s from above. */
  157         return (K <<= p);
  158 }
  159 
  160 /*
  161  * Compute the new cwnd value using an implementation of eqn 1 from the I-D.
  162  * Thanks to Kip Macy for help debugging this function.
  163  *
  164  * XXXLAS: Characterise bounds for overflow.
  165  */
  166 static __inline unsigned long
  167 cubic_cwnd(int ticks_since_cong, unsigned long wmax, uint32_t smss, int64_t K)
  168 {
  169         int64_t cwnd;
  170 
  171         /* K is in fixed point form with CUBIC_SHIFT worth of precision. */
  172 
  173         /* t - K, with CUBIC_SHIFT worth of precision. */
  174         cwnd = ((int64_t)(ticks_since_cong << CUBIC_SHIFT) - (K * hz)) / hz;
  175 
  176         /* (t - K)^3, with CUBIC_SHIFT^3 worth of precision. */
  177         cwnd *= (cwnd * cwnd);
  178 
  179         /*
  180          * C(t - K)^3 + wmax
  181          * The down shift by CUBIC_SHIFT_4 is because cwnd has 4 lots of
  182          * CUBIC_SHIFT included in the value. 3 from the cubing of cwnd above,
  183          * and an extra from multiplying through by CUBIC_C_FACTOR.
  184          */
  185         cwnd = ((cwnd * CUBIC_C_FACTOR * smss) >> CUBIC_SHIFT_4) + wmax;
  186 
  187         return ((unsigned long)cwnd);
  188 }
  189 
  190 /*
  191  * Compute an approximation of the NewReno cwnd some number of ticks after a
  192  * congestion event. RTT should be the average RTT estimate for the path
  193  * measured over the previous congestion epoch and wmax is the value of cwnd at
  194  * the last congestion event. The "TCP friendly" concept in the CUBIC I-D is
  195  * rather tricky to understand and it turns out this function is not required.
  196  * It is left here for reference.
  197  */
  198 static __inline unsigned long
  199 reno_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
  200     uint32_t smss)
  201 {
  202 
  203         /*
  204          * For NewReno, beta = 0.5, therefore: W_tcp(t) = wmax*0.5 + t/RTT
  205          * W_tcp(t) deals with cwnd/wmax in pkts, so because our cwnd is in
  206          * bytes, we have to multiply by smss.
  207          */
  208         return (((wmax * RENO_BETA) + (((ticks_since_cong * smss)
  209             << CUBIC_SHIFT) / rtt_ticks)) >> CUBIC_SHIFT);
  210 }
  211 
  212 /*
  213  * Compute an approximation of the "TCP friendly" cwnd some number of ticks
  214  * after a congestion event that is designed to yield the same average cwnd as
  215  * NewReno while using CUBIC's beta of 0.8. RTT should be the average RTT
  216  * estimate for the path measured over the previous congestion epoch and wmax is
  217  * the value of cwnd at the last congestion event.
  218  */
  219 static __inline unsigned long
  220 tf_cwnd(int ticks_since_cong, int rtt_ticks, unsigned long wmax,
  221     uint32_t smss)
  222 {
  223 
  224         /* Equation 4 of I-D. */
  225         return (((wmax * CUBIC_BETA) + (((THREE_X_PT2 * ticks_since_cong *
  226             smss) << CUBIC_SHIFT) / TWO_SUB_PT2 / rtt_ticks)) >> CUBIC_SHIFT);
  227 }
  228 
  229 #endif /* _NETINET_CC_CUBIC_H_ */

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