xref: /freebsd/sys/netinet/cc/cc_cubic.h (revision 3f0164abf32b9b761e0a2cb4bdca3a8b84f156d4)
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  * $FreeBSD$
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_ */
230