1 | // SPDX-License-Identifier: GPL-2.0-or-later |
2 | /* |
3 | * Copyright (c) 2005 The University of Waikato, Hamilton, New Zealand. |
4 | * Copyright (c) 2005 Ian McDonald <ian.mcdonald@jandi.co.nz> |
5 | * Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br> |
6 | * Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon |
7 | */ |
8 | |
9 | #include <linux/module.h> |
10 | #include "../../dccp.h" |
11 | #include "tfrc.h" |
12 | |
13 | #define TFRC_CALC_X_ARRSIZE 500 |
14 | #define TFRC_CALC_X_SPLIT 50000 /* 0.05 * 1000000, details below */ |
15 | #define TFRC_SMALLEST_P (TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE) |
16 | |
17 | /* |
18 | TFRC TCP Reno Throughput Equation Lookup Table for f(p) |
19 | |
20 | The following two-column lookup table implements a part of the TCP throughput |
21 | equation from [RFC 3448, sec. 3.1]: |
22 | |
23 | s |
24 | X_calc = -------------------------------------------------------------- |
25 | R * sqrt(2*b*p/3) + (3 * t_RTO * sqrt(3*b*p/8) * (p + 32*p^3)) |
26 | |
27 | Where: |
28 | X is the transmit rate in bytes/second |
29 | s is the packet size in bytes |
30 | R is the round trip time in seconds |
31 | p is the loss event rate, between 0 and 1.0, of the number of loss |
32 | events as a fraction of the number of packets transmitted |
33 | t_RTO is the TCP retransmission timeout value in seconds |
34 | b is the number of packets acknowledged by a single TCP ACK |
35 | |
36 | We can assume that b = 1 and t_RTO is 4 * R. The equation now becomes: |
37 | |
38 | s |
39 | X_calc = ------------------------------------------------------- |
40 | R * sqrt(p*2/3) + (12 * R * sqrt(p*3/8) * (p + 32*p^3)) |
41 | |
42 | which we can break down into: |
43 | |
44 | s |
45 | X_calc = --------- |
46 | R * f(p) |
47 | |
48 | where f(p) is given for 0 < p <= 1 by: |
49 | |
50 | f(p) = sqrt(2*p/3) + 12 * sqrt(3*p/8) * (p + 32*p^3) |
51 | |
52 | Since this is kernel code, floating-point arithmetic is avoided in favour of |
53 | integer arithmetic. This means that nearly all fractional parameters are |
54 | scaled by 1000000: |
55 | * the parameters p and R |
56 | * the return result f(p) |
57 | The lookup table therefore actually tabulates the following function g(q): |
58 | |
59 | g(q) = 1000000 * f(q/1000000) |
60 | |
61 | Hence, when p <= 1, q must be less than or equal to 1000000. To achieve finer |
62 | granularity for the practically more relevant case of small values of p (up to |
63 | 5%), the second column is used; the first one ranges up to 100%. This split |
64 | corresponds to the value of q = TFRC_CALC_X_SPLIT. At the same time this also |
65 | determines the smallest resolution possible with this lookup table: |
66 | |
67 | TFRC_SMALLEST_P = TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE |
68 | |
69 | The entire table is generated by: |
70 | for(i=0; i < TFRC_CALC_X_ARRSIZE; i++) { |
71 | lookup[i][0] = g((i+1) * 1000000/TFRC_CALC_X_ARRSIZE); |
72 | lookup[i][1] = g((i+1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE); |
73 | } |
74 | |
75 | With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1, |
76 | lookup[0][0] = g(1000000/(M+1)) = 1000000 * f(0.2%) |
77 | lookup[M][0] = g(1000000) = 1000000 * f(100%) |
78 | lookup[0][1] = g(TFRC_SMALLEST_P) = 1000000 * f(0.01%) |
79 | lookup[M][1] = g(TFRC_CALC_X_SPLIT) = 1000000 * f(5%) |
80 | |
81 | In summary, the two columns represent f(p) for the following ranges: |
82 | * The first column is for 0.002 <= p <= 1.0 |
83 | * The second column is for 0.0001 <= p <= 0.05 |
84 | Where the columns overlap, the second (finer-grained) is given preference, |
85 | i.e. the first column is used only for p >= 0.05. |
86 | */ |
87 | static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = { |
88 | { 37172, 8172 }, |
89 | { 53499, 11567 }, |
90 | { 66664, 14180 }, |
91 | { 78298, 16388 }, |
92 | { 89021, 18339 }, |
93 | { 99147, 20108 }, |
94 | { 108858, 21738 }, |
95 | { 118273, 23260 }, |
96 | { 127474, 24693 }, |
97 | { 136520, 26052 }, |
98 | { 145456, 27348 }, |
99 | { 154316, 28589 }, |
100 | { 163130, 29783 }, |
101 | { 171919, 30935 }, |
102 | { 180704, 32049 }, |
103 | { 189502, 33130 }, |
104 | { 198328, 34180 }, |
105 | { 207194, 35202 }, |
106 | { 216114, 36198 }, |
107 | { 225097, 37172 }, |
108 | { 234153, 38123 }, |
109 | { 243294, 39055 }, |
110 | { 252527, 39968 }, |
111 | { 261861, 40864 }, |
112 | { 271305, 41743 }, |
113 | { 280866, 42607 }, |
114 | { 290553, 43457 }, |
115 | { 300372, 44293 }, |
116 | { 310333, 45117 }, |
117 | { 320441, 45929 }, |
118 | { 330705, 46729 }, |
119 | { 341131, 47518 }, |
120 | { 351728, 48297 }, |
121 | { 362501, 49066 }, |
122 | { 373460, 49826 }, |
123 | { 384609, 50577 }, |
124 | { 395958, 51320 }, |
125 | { 407513, 52054 }, |
126 | { 419281, 52780 }, |
127 | { 431270, 53499 }, |
128 | { 443487, 54211 }, |
129 | { 455940, 54916 }, |
130 | { 468635, 55614 }, |
131 | { 481581, 56306 }, |
132 | { 494785, 56991 }, |
133 | { 508254, 57671 }, |
134 | { 521996, 58345 }, |
135 | { 536019, 59014 }, |
136 | { 550331, 59677 }, |
137 | { 564939, 60335 }, |
138 | { 579851, 60988 }, |
139 | { 595075, 61636 }, |
140 | { 610619, 62279 }, |
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142 | { 642700, 63553 }, |
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177 | { 1489430, 83750 }, |
178 | { 1523671, 84284 }, |
179 | { 1558593, 84817 }, |
180 | { 1594205, 85348 }, |
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182 | { 1667543, 86406 }, |
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572 | { 219196666, 264211 }, |
573 | { 220748520, 264682 }, |
574 | { 222308282, 265153 }, |
575 | { 223875978, 265625 }, |
576 | { 225451630, 266097 }, |
577 | { 227035265, 266569 }, |
578 | { 228626905, 267041 }, |
579 | { 230226576, 267514 }, |
580 | { 231834302, 267986 }, |
581 | { 233450107, 268460 }, |
582 | { 235074016, 268933 }, |
583 | { 236706054, 269407 }, |
584 | { 238346244, 269881 }, |
585 | { 239994613, 270355 }, |
586 | { 241651183, 270830 }, |
587 | { 243315981, 271305 } |
588 | }; |
589 | |
590 | /* return largest index i such that fval <= lookup[i][small] */ |
591 | static inline u32 tfrc_binsearch(u32 fval, u8 small) |
592 | { |
593 | u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1; |
594 | |
595 | while (low < high) { |
596 | try = (low + high) / 2; |
597 | if (fval <= tfrc_calc_x_lookup[try][small]) |
598 | high = try; |
599 | else |
600 | low = try + 1; |
601 | } |
602 | return high; |
603 | } |
604 | |
605 | /** |
606 | * tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448 |
607 | * @s: packet size in bytes |
608 | * @R: RTT scaled by 1000000 (i.e., microseconds) |
609 | * @p: loss ratio estimate scaled by 1000000 |
610 | * |
611 | * Returns X_calc in bytes per second (not scaled). |
612 | */ |
613 | u32 tfrc_calc_x(u16 s, u32 R, u32 p) |
614 | { |
615 | u16 index; |
616 | u32 f; |
617 | u64 result; |
618 | |
619 | /* check against invalid parameters and divide-by-zero */ |
620 | BUG_ON(p > 1000000); /* p must not exceed 100% */ |
621 | BUG_ON(p == 0); /* f(0) = 0, divide by zero */ |
622 | if (R == 0) { /* possible divide by zero */ |
623 | DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc." ); |
624 | return ~0U; |
625 | } |
626 | |
627 | if (p <= TFRC_CALC_X_SPLIT) { /* 0.0000 < p <= 0.05 */ |
628 | if (p < TFRC_SMALLEST_P) { /* 0.0000 < p < 0.0001 */ |
629 | DCCP_WARN("Value of p (%d) below resolution. " |
630 | "Substituting %d\n" , p, TFRC_SMALLEST_P); |
631 | index = 0; |
632 | } else /* 0.0001 <= p <= 0.05 */ |
633 | index = p/TFRC_SMALLEST_P - 1; |
634 | |
635 | f = tfrc_calc_x_lookup[index][1]; |
636 | |
637 | } else { /* 0.05 < p <= 1.00 */ |
638 | index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1; |
639 | |
640 | f = tfrc_calc_x_lookup[index][0]; |
641 | } |
642 | |
643 | /* |
644 | * Compute X = s/(R*f(p)) in bytes per second. |
645 | * Since f(p) and R are both scaled by 1000000, we need to multiply by |
646 | * 1000000^2. To avoid overflow, the result is computed in two stages. |
647 | * This works under almost all reasonable operational conditions, for a |
648 | * wide range of parameters. Yet, should some strange combination of |
649 | * parameters result in overflow, the use of scaled_div32 will catch |
650 | * this and return UINT_MAX - which is a logically adequate consequence. |
651 | */ |
652 | result = scaled_div(a: s, b: R); |
653 | return scaled_div32(a: result, b: f); |
654 | } |
655 | |
656 | /** |
657 | * tfrc_calc_x_reverse_lookup - try to find p given f(p) |
658 | * @fvalue: function value to match, scaled by 1000000 |
659 | * |
660 | * Returns closest match for p, also scaled by 1000000 |
661 | */ |
662 | u32 tfrc_calc_x_reverse_lookup(u32 fvalue) |
663 | { |
664 | int index; |
665 | |
666 | if (fvalue == 0) /* f(p) = 0 whenever p = 0 */ |
667 | return 0; |
668 | |
669 | /* Error cases. */ |
670 | if (fvalue < tfrc_calc_x_lookup[0][1]) { |
671 | DCCP_WARN("fvalue %u smaller than resolution\n" , fvalue); |
672 | return TFRC_SMALLEST_P; |
673 | } |
674 | if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) { |
675 | DCCP_WARN("fvalue %u exceeds bounds!\n" , fvalue); |
676 | return 1000000; |
677 | } |
678 | |
679 | if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) { |
680 | index = tfrc_binsearch(fval: fvalue, small: 1); |
681 | return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE; |
682 | } |
683 | |
684 | /* else ... it must be in the coarse-grained column */ |
685 | index = tfrc_binsearch(fval: fvalue, small: 0); |
686 | return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE; |
687 | } |
688 | |
689 | /** |
690 | * tfrc_invert_loss_event_rate - Compute p so that 10^6 corresponds to 100% |
691 | * @loss_event_rate: loss event rate to invert |
692 | * When @loss_event_rate is large, there is a chance that p is truncated to 0. |
693 | * To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0. |
694 | */ |
695 | u32 tfrc_invert_loss_event_rate(u32 loss_event_rate) |
696 | { |
697 | if (loss_event_rate == UINT_MAX) /* see RFC 4342, 8.5 */ |
698 | return 0; |
699 | if (unlikely(loss_event_rate == 0)) /* map 1/0 into 100% */ |
700 | return 1000000; |
701 | return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P); |
702 | } |
703 | |