1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
|
/*
* Copyright (c) 2005 The University of Waikato, Hamilton, New Zealand.
* Copyright (c) 2005 Ian McDonald <ian.mcdonald@jandi.co.nz>
* Copyright (c) 2005 Arnaldo Carvalho de Melo <acme@conectiva.com.br>
* Copyright (c) 2003 Nils-Erik Mattsson, Joacim Haggmark, Magnus Erixzon
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*/
#include <linux/module.h>
#include "../../dccp.h"
#include "tfrc.h"
#define TFRC_CALC_X_ARRSIZE 500
#define TFRC_CALC_X_SPLIT 50000 /* 0.05 * 1000000, details below */
#define TFRC_SMALLEST_P (TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE)
/*
TFRC TCP Reno Throughput Equation Lookup Table for f(p)
The following two-column lookup table implements a part of the TCP throughput
equation from [RFC 3448, sec. 3.1]:
s
X_calc = --------------------------------------------------------------
R * sqrt(2*b*p/3) + (3 * t_RTO * sqrt(3*b*p/8) * (p + 32*p^3))
Where:
X is the transmit rate in bytes/second
s is the packet size in bytes
R is the round trip time in seconds
p is the loss event rate, between 0 and 1.0, of the number of loss
events as a fraction of the number of packets transmitted
t_RTO is the TCP retransmission timeout value in seconds
b is the number of packets acknowledged by a single TCP ACK
We can assume that b = 1 and t_RTO is 4 * R. The equation now becomes:
s
X_calc = -------------------------------------------------------
R * sqrt(p*2/3) + (12 * R * sqrt(p*3/8) * (p + 32*p^3))
which we can break down into:
s
X_calc = ---------
R * f(p)
where f(p) is given for 0 < p <= 1 by:
f(p) = sqrt(2*p/3) + 12 * sqrt(3*p/8) * (p + 32*p^3)
Since this is kernel code, floating-point arithmetic is avoided in favour of
integer arithmetic. This means that nearly all fractional parameters are
scaled by 1000000:
* the parameters p and R
* the return result f(p)
The lookup table therefore actually tabulates the following function g(q):
g(q) = 1000000 * f(q/1000000)
Hence, when p <= 1, q must be less than or equal to 1000000. To achieve finer
granularity for the practically more relevant case of small values of p (up to
5%), the second column is used; the first one ranges up to 100%. This split
corresponds to the value of q = TFRC_CALC_X_SPLIT. At the same time this also
determines the smallest resolution possible with this lookup table:
TFRC_SMALLEST_P = TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE
The entire table is generated by:
for(i=0; i < TFRC_CALC_X_ARRSIZE; i++) {
lookup[i][0] = g((i+1) * 1000000/TFRC_CALC_X_ARRSIZE);
lookup[i][1] = g((i+1) * TFRC_CALC_X_SPLIT/TFRC_CALC_X_ARRSIZE);
}
With the given configuration, we have, with M = TFRC_CALC_X_ARRSIZE-1,
lookup[0][0] = g(1000000/(M+1)) = 1000000 * f(0.2%)
lookup[M][0] = g(1000000) = 1000000 * f(100%)
lookup[0][1] = g(TFRC_SMALLEST_P) = 1000000 * f(0.01%)
lookup[M][1] = g(TFRC_CALC_X_SPLIT) = 1000000 * f(5%)
In summary, the two columns represent f(p) for the following ranges:
* The first column is for 0.002 <= p <= 1.0
* The second column is for 0.0001 <= p <= 0.05
Where the columns overlap, the second (finer-grained) is given preference,
i.e. the first column is used only for p >= 0.05.
*/
static const u32 tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE][2] = {
{ 37172, 8172 },
{ 53499, 11567 },
{ 66664, 14180 },
{ 78298, 16388 },
{ 89021, 18339 },
{ 99147, 20108 },
{ 108858, 21738 },
{ 118273, 23260 },
{ 127474, 24693 },
{ 136520, 26052 },
{ 145456, 27348 },
{ 154316, 28589 },
{ 163130, 29783 },
{ 171919, 30935 },
{ 180704, 32049 },
{ 189502, 33130 },
{ 198328, 34180 },
{ 207194, 35202 },
{ 216114, 36198 },
{ 225097, 37172 },
{ 234153, 38123 },
{ 243294, 39055 },
{ 252527, 39968 },
{ 261861, 40864 },
{ 271305, 41743 },
{ 280866, 42607 },
{ 290553, 43457 },
{ 300372, 44293 },
{ 310333, 45117 },
{ 320441, 45929 },
{ 330705, 46729 },
{ 341131, 47518 },
{ 351728, 48297 },
{ 362501, 49066 },
{ 373460, 49826 },
{ 384609, 50577 },
{ 395958, 51320 },
{ 407513, 52054 },
{ 419281, 52780 },
{ 431270, 53499 },
{ 443487, 54211 },
{ 455940, 54916 },
{ 468635, 55614 },
{ 481581, 56306 },
{ 494785, 56991 },
{ 508254, 57671 },
{ 521996, 58345 },
{ 536019, 59014 },
{ 550331, 59677 },
{ 564939, 60335 },
{ 579851, 60988 },
{ 595075, 61636 },
{ 610619, 62279 },
{ 626491, 62918 },
{ 642700, 63553 },
{ 659253, 64183 },
{ 676158, 64809 },
{ 693424, 65431 },
{ 711060, 66050 },
{ 729073, 66664 },
{ 747472, 67275 },
{ 766266, 67882 },
{ 785464, 68486 },
{ 805073, 69087 },
{ 825103, 69684 },
{ 845562, 70278 },
{ 866460, 70868 },
{ 887805, 71456 },
{ 909606, 72041 },
{ 931873, 72623 },
{ 954614, 73202 },
{ 977839, 73778 },
{ 1001557, 74352 },
{ 1025777, 74923 },
{ 1050508, 75492 },
{ 1075761, 76058 },
{ 1101544, 76621 },
{ 1127867, 77183 },
{ 1154739, 77741 },
{ 1182172, 78298 },
{ 1210173, 78852 },
{ 1238753, 79405 },
{ 1267922, 79955 },
{ 1297689, 80503 },
{ 1328066, 81049 },
{ 1359060, 81593 },
{ 1390684, 82135 },
{ 1422947, 82675 },
{ 1455859, 83213 },
{ 1489430, 83750 },
{ 1523671, 84284 },
{ 1558593, 84817 },
{ 1594205, 85348 },
{ 1630518, 85878 },
{ 1667543, 86406 },
{ 1705290, 86932 },
{ 1743770, 87457 },
{ 1782994, 87980 },
{ 1822973, 88501 },
{ 1863717, 89021 },
{ 1905237, 89540 },
{ 1947545, 90057 },
{ 1990650, 90573 },
{ 2034566, 91087 },
{ 2079301, 91600 },
{ 2124869, 92111 },
{ 2171279, 92622 },
{ 2218543, 93131 },
{ 2266673, 93639 },
{ 2315680, 94145 },
{ 2365575, 94650 },
{ 2416371, 95154 },
{ 2468077, 95657 },
{ 2520707, 96159 },
{ 2574271, 96660 },
{ 2628782, 97159 },
{ 2684250, 97658 },
{ 2740689, 98155 },
{ 2798110, 98651 },
{ 2856524, 99147 },
{ 2915944, 99641 },
{ 2976382, 100134 },
{ 3037850, 100626 },
{ 3100360, 101117 },
{ 3163924, 101608 },
{ 3228554, 102097 },
{ 3294263, 102586 },
{ 3361063, 103073 },
{ 3428966, 103560 },
{ 3497984, 104045 },
{ 3568131, 104530 },
{ 3639419, 105014 },
{ 3711860, 105498 },
{ 3785467, 105980 },
{ 3860253, 106462 },
{ 3936229, 106942 },
{ 4013410, 107422 },
{ 4091808, 107902 },
{ 4171435, 108380 },
{ 4252306, 108858 },
{ 4334431, 109335 },
{ 4417825, 109811 },
{ 4502501, 110287 },
{ 4588472, 110762 },
{ 4675750, 111236 },
{ 4764349, 111709 },
{ 4854283, 112182 },
{ 4945564, 112654 },
{ 5038206, 113126 },
{ 5132223, 113597 },
{ 5227627, 114067 },
{ 5324432, 114537 },
{ 5422652, 115006 },
{ 5522299, 115474 },
{ 5623389, 115942 },
{ 5725934, 116409 },
{ 5829948, 116876 },
{ 5935446, 117342 },
{ 6042439, 117808 },
{ 6150943, 118273 },
{ 6260972, 118738 },
{ 6372538, 119202 },
{ 6485657, 119665 },
{ 6600342, 120128 },
{ 6716607, 120591 },
{ 6834467, 121053 },
{ 6953935, 121514 },
{ 7075025, 121976 },
{ 7197752, 122436 },
{ 7322131, 122896 },
{ 7448175, 123356 },
{ 7575898, 123815 },
{ 7705316, 124274 },
{ 7836442, 124733 },
{ 7969291, 125191 },
{ 8103877, 125648 },
{ 8240216, 126105 },
{ 8378321, 126562 },
{ 8518208, 127018 },
{ 8659890, 127474 },
{ 8803384, 127930 },
{ 8948702, 128385 },
{ 9095861, 128840 },
{ 9244875, 129294 },
{ 9395760, 129748 },
{ 9548529, 130202 },
{ 9703198, 130655 },
{ 9859782, 131108 },
{ 10018296, 131561 },
{ 10178755, 132014 },
{ 10341174, 132466 },
{ 10505569, 132917 },
{ 10671954, 133369 },
{ 10840345, 133820 },
{ 11010757, 134271 },
{ 11183206, 134721 },
{ 11357706, 135171 },
{ 11534274, 135621 },
{ 11712924, 136071 },
{ 11893673, 136520 },
{ 12076536, 136969 },
{ 12261527, 137418 },
{ 12448664, 137867 },
{ 12637961, 138315 },
{ 12829435, 138763 },
{ 13023101, 139211 },
{ 13218974, 139658 },
{ 13417071, 140106 },
{ 13617407, 140553 },
{ 13819999, 140999 },
{ 14024862, 141446 },
{ 14232012, 141892 },
{ 14441465, 142339 },
{ 14653238, 142785 },
{ 14867346, 143230 },
{ 15083805, 143676 },
{ 15302632, 144121 },
{ 15523842, 144566 },
{ 15747453, 145011 },
{ 15973479, 145456 },
{ 16201939, 145900 },
{ 16432847, 146345 },
{ 16666221, 146789 },
{ 16902076, 147233 },
{ 17140429, 147677 },
{ 17381297, 148121 },
{ 17624696, 148564 },
{ 17870643, 149007 },
{ 18119154, 149451 },
{ 18370247, 149894 },
{ 18623936, 150336 },
{ 18880241, 150779 },
{ 19139176, 151222 },
{ 19400759, 151664 },
{ 19665007, 152107 },
{ 19931936, 152549 },
{ 20201564, 152991 },
{ 20473907, 153433 },
{ 20748982, 153875 },
{ 21026807, 154316 },
{ 21307399, 154758 },
{ 21590773, 155199 },
{ 21876949, 155641 },
{ 22165941, 156082 },
{ 22457769, 156523 },
{ 22752449, 156964 },
{ 23049999, 157405 },
{ 23350435, 157846 },
{ 23653774, 158287 },
{ 23960036, 158727 },
{ 24269236, 159168 },
{ 24581392, 159608 },
{ 24896521, 160049 },
{ 25214642, 160489 },
{ 25535772, 160929 },
{ 25859927, 161370 },
{ 26187127, 161810 },
{ 26517388, 162250 },
{ 26850728, 162690 },
{ 27187165, 163130 },
{ 27526716, 163569 },
{ 27869400, 164009 },
{ 28215234, 164449 },
{ 28564236, 164889 },
{ 28916423, 165328 },
{ 29271815, 165768 },
{ 29630428, 166208 },
{ 29992281, 166647 },
{ 30357392, 167087 },
{ 30725779, 167526 },
{ 31097459, 167965 },
{ 31472452, 168405 },
{ 31850774, 168844 },
{ 32232445, 169283 },
{ 32617482, 169723 },
{ 33005904, 170162 },
{ 33397730, 170601 },
{ 33792976, 171041 },
{ 34191663, 171480 },
{ 34593807, 171919 },
{ 34999428, 172358 },
{ 35408544, 172797 },
{ 35821174, 173237 },
{ 36237335, 173676 },
{ 36657047, 174115 },
{ 37080329, 174554 },
{ 37507197, 174993 },
{ 37937673, 175433 },
{ 38371773, 175872 },
{ 38809517, 176311 },
{ 39250924, 176750 },
{ 39696012, 177190 },
{ 40144800, 177629 },
{ 40597308, 178068 },
{ 41053553, 178507 },
{ 41513554, 178947 },
{ 41977332, 179386 },
{ 42444904, 179825 },
{ 42916290, 180265 },
{ 43391509, 180704 },
{ 43870579, 181144 },
{ 44353520, 181583 },
{ 44840352, 182023 },
{ 45331092, 182462 },
{ 45825761, 182902 },
{ 46324378, 183342 },
{ 46826961, 183781 },
{ 47333531, 184221 },
{ 47844106, 184661 },
{ 48358706, 185101 },
{ 48877350, 185541 },
{ 49400058, 185981 },
{ 49926849, 186421 },
{ 50457743, 186861 },
{ 50992759, 187301 },
{ 51531916, 187741 },
{ 52075235, 188181 },
{ 52622735, 188622 },
{ 53174435, 189062 },
{ 53730355, 189502 },
{ 54290515, 189943 },
{ 54854935, 190383 },
{ 55423634, 190824 },
{ 55996633, 191265 },
{ 56573950, 191706 },
{ 57155606, 192146 },
{ 57741621, 192587 },
{ 58332014, 193028 },
{ 58926806, 193470 },
{ 59526017, 193911 },
{ 60129666, 194352 },
{ 60737774, 194793 },
{ 61350361, 195235 },
{ 61967446, 195677 },
{ 62589050, 196118 },
{ 63215194, 196560 },
{ 63845897, 197002 },
{ 64481179, 197444 },
{ 65121061, 197886 },
{ 65765563, 198328 },
{ 66414705, 198770 },
{ 67068508, 199213 },
{ 67726992, 199655 },
{ 68390177, 200098 },
{ 69058085, 200540 },
{ 69730735, 200983 },
{ 70408147, 201426 },
{ 71090343, 201869 },
{ 71777343, 202312 },
{ 72469168, 202755 },
{ 73165837, 203199 },
{ 73867373, 203642 },
{ 74573795, 204086 },
{ 75285124, 204529 },
{ 76001380, 204973 },
{ 76722586, 205417 },
{ 77448761, 205861 },
{ 78179926, 206306 },
{ 78916102, 206750 },
{ 79657310, 207194 },
{ 80403571, 207639 },
{ 81154906, 208084 },
{ 81911335, 208529 },
{ 82672880, 208974 },
{ 83439562, 209419 },
{ 84211402, 209864 },
{ 84988421, 210309 },
{ 85770640, 210755 },
{ 86558080, 211201 },
{ 87350762, 211647 },
{ 88148708, 212093 },
{ 88951938, 212539 },
{ 89760475, 212985 },
{ 90574339, 213432 },
{ 91393551, 213878 },
{ 92218133, 214325 },
{ 93048107, 214772 },
{ 93883493, 215219 },
{ 94724314, 215666 },
{ 95570590, 216114 },
{ 96422343, 216561 },
{ 97279594, 217009 },
{ 98142366, 217457 },
{ 99010679, 217905 },
{ 99884556, 218353 },
{ 100764018, 218801 },
{ 101649086, 219250 },
{ 102539782, 219698 },
{ 103436128, 220147 },
{ 104338146, 220596 },
{ 105245857, 221046 },
{ 106159284, 221495 },
{ 107078448, 221945 },
{ 108003370, 222394 },
{ 108934074, 222844 },
{ 109870580, 223294 },
{ 110812910, 223745 },
{ 111761087, 224195 },
{ 112715133, 224646 },
{ 113675069, 225097 },
{ 114640918, 225548 },
{ 115612702, 225999 },
{ 116590442, 226450 },
{ 117574162, 226902 },
{ 118563882, 227353 },
{ 119559626, 227805 },
{ 120561415, 228258 },
{ 121569272, 228710 },
{ 122583219, 229162 },
{ 123603278, 229615 },
{ 124629471, 230068 },
{ 125661822, 230521 },
{ 126700352, 230974 },
{ 127745083, 231428 },
{ 128796039, 231882 },
{ 129853241, 232336 },
{ 130916713, 232790 },
{ 131986475, 233244 },
{ 133062553, 233699 },
{ 134144966, 234153 },
{ 135233739, 234608 },
{ 136328894, 235064 },
{ 137430453, 235519 },
{ 138538440, 235975 },
{ 139652876, 236430 },
{ 140773786, 236886 },
{ 141901190, 237343 },
{ 143035113, 237799 },
{ 144175576, 238256 },
{ 145322604, 238713 },
{ 146476218, 239170 },
{ 147636442, 239627 },
{ 148803298, 240085 },
{ 149976809, 240542 },
{ 151156999, 241000 },
{ 152343890, 241459 },
{ 153537506, 241917 },
{ 154737869, 242376 },
{ 155945002, 242835 },
{ 157158929, 243294 },
{ 158379673, 243753 },
{ 159607257, 244213 },
{ 160841704, 244673 },
{ 162083037, 245133 },
{ 163331279, 245593 },
{ 164586455, 246054 },
{ 165848586, 246514 },
{ 167117696, 246975 },
{ 168393810, 247437 },
{ 169676949, 247898 },
{ 170967138, 248360 },
{ 172264399, 248822 },
{ 173568757, 249284 },
{ 174880235, 249747 },
{ 176198856, 250209 },
{ 177524643, 250672 },
{ 178857621, 251136 },
{ 180197813, 251599 },
{ 181545242, 252063 },
{ 182899933, 252527 },
{ 184261908, 252991 },
{ 185631191, 253456 },
{ 187007807, 253920 },
{ 188391778, 254385 },
{ 189783129, 254851 },
{ 191181884, 255316 },
{ 192588065, 255782 },
{ 194001698, 256248 },
{ 195422805, 256714 },
{ 196851411, 257181 },
{ 198287540, 257648 },
{ 199731215, 258115 },
{ 201182461, 258582 },
{ 202641302, 259050 },
{ 204107760, 259518 },
{ 205581862, 259986 },
{ 207063630, 260454 },
{ 208553088, 260923 },
{ 210050262, 261392 },
{ 211555174, 261861 },
{ 213067849, 262331 },
{ 214588312, 262800 },
{ 216116586, 263270 },
{ 217652696, 263741 },
{ 219196666, 264211 },
{ 220748520, 264682 },
{ 222308282, 265153 },
{ 223875978, 265625 },
{ 225451630, 266097 },
{ 227035265, 266569 },
{ 228626905, 267041 },
{ 230226576, 267514 },
{ 231834302, 267986 },
{ 233450107, 268460 },
{ 235074016, 268933 },
{ 236706054, 269407 },
{ 238346244, 269881 },
{ 239994613, 270355 },
{ 241651183, 270830 },
{ 243315981, 271305 }
};
/* return largest index i such that fval <= lookup[i][small] */
static inline u32 tfrc_binsearch(u32 fval, u8 small)
{
u32 try, low = 0, high = TFRC_CALC_X_ARRSIZE - 1;
while (low < high) {
try = (low + high) / 2;
if (fval <= tfrc_calc_x_lookup[try][small])
high = try;
else
low = try + 1;
}
return high;
}
/**
* tfrc_calc_x - Calculate the send rate as per section 3.1 of RFC3448
* @s: packet size in bytes
* @R: RTT scaled by 1000000 (i.e., microseconds)
* @p: loss ratio estimate scaled by 1000000
*
* Returns X_calc in bytes per second (not scaled).
*/
u32 tfrc_calc_x(u16 s, u32 R, u32 p)
{
u16 index;
u32 f;
u64 result;
/* check against invalid parameters and divide-by-zero */
BUG_ON(p > 1000000); /* p must not exceed 100% */
BUG_ON(p == 0); /* f(0) = 0, divide by zero */
if (R == 0) { /* possible divide by zero */
DCCP_CRIT("WARNING: RTT is 0, returning maximum X_calc.");
return ~0U;
}
if (p <= TFRC_CALC_X_SPLIT) { /* 0.0000 < p <= 0.05 */
if (p < TFRC_SMALLEST_P) { /* 0.0000 < p < 0.0001 */
DCCP_WARN("Value of p (%d) below resolution. "
"Substituting %d\n", p, TFRC_SMALLEST_P);
index = 0;
} else /* 0.0001 <= p <= 0.05 */
index = p/TFRC_SMALLEST_P - 1;
f = tfrc_calc_x_lookup[index][1];
} else { /* 0.05 < p <= 1.00 */
index = p/(1000000/TFRC_CALC_X_ARRSIZE) - 1;
f = tfrc_calc_x_lookup[index][0];
}
/*
* Compute X = s/(R*f(p)) in bytes per second.
* Since f(p) and R are both scaled by 1000000, we need to multiply by
* 1000000^2. To avoid overflow, the result is computed in two stages.
* This works under almost all reasonable operational conditions, for a
* wide range of parameters. Yet, should some strange combination of
* parameters result in overflow, the use of scaled_div32 will catch
* this and return UINT_MAX - which is a logically adequate consequence.
*/
result = scaled_div(s, R);
return scaled_div32(result, f);
}
/**
* tfrc_calc_x_reverse_lookup - try to find p given f(p)
* @fvalue: function value to match, scaled by 1000000
*
* Returns closest match for p, also scaled by 1000000
*/
u32 tfrc_calc_x_reverse_lookup(u32 fvalue)
{
int index;
if (fvalue == 0) /* f(p) = 0 whenever p = 0 */
return 0;
/* Error cases. */
if (fvalue < tfrc_calc_x_lookup[0][1]) {
DCCP_WARN("fvalue %u smaller than resolution\n", fvalue);
return TFRC_SMALLEST_P;
}
if (fvalue > tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][0]) {
DCCP_WARN("fvalue %u exceeds bounds!\n", fvalue);
return 1000000;
}
if (fvalue <= tfrc_calc_x_lookup[TFRC_CALC_X_ARRSIZE - 1][1]) {
index = tfrc_binsearch(fvalue, 1);
return (index + 1) * TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE;
}
/* else ... it must be in the coarse-grained column */
index = tfrc_binsearch(fvalue, 0);
return (index + 1) * 1000000 / TFRC_CALC_X_ARRSIZE;
}
/**
* tfrc_invert_loss_event_rate - Compute p so that 10^6 corresponds to 100%
* When @loss_event_rate is large, there is a chance that p is truncated to 0.
* To avoid re-entering slow-start in that case, we set p = TFRC_SMALLEST_P > 0.
*/
u32 tfrc_invert_loss_event_rate(u32 loss_event_rate)
{
if (loss_event_rate == UINT_MAX) /* see RFC 4342, 8.5 */
return 0;
if (unlikely(loss_event_rate == 0)) /* map 1/0 into 100% */
return 1000000;
return max_t(u32, scaled_div(1, loss_event_rate), TFRC_SMALLEST_P);
}
|
