summaryrefslogtreecommitdiff
path: root/net/ipv4/tcp_cubic.c
blob: 59155af9de5d40ccf73daa8496cf9fd79cd2a987 (plain)
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
/*
 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
 * Home page:
 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
 * This is from the implementation of CUBIC TCP in
 * Sangtae Ha, Injong Rhee and Lisong Xu,
 *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
 *  in ACM SIGOPS Operating System Review, July 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
 *
 * CUBIC integrates a new slow start algorithm, called HyStart.
 * The details of HyStart are presented in
 *  Sangtae Ha and Injong Rhee,
 *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
 * Available from:
 *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
 *
 * All testing results are available from:
 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
 *
 * Unless CUBIC is enabled and congestion window is large
 * this behaves the same as the original Reno.
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/math64.h>
#include <net/tcp.h>

#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
					 * max_cwnd = snd_cwnd * beta
					 */
#define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */

/* Two methods of hybrid slow start */
#define HYSTART_ACK_TRAIN	0x1
#define HYSTART_DELAY		0x2

/* Number of delay samples for detecting the increase of delay */
#define HYSTART_MIN_SAMPLES	8
#define HYSTART_DELAY_MIN	(4U<<3)
#define HYSTART_DELAY_MAX	(16U<<3)
#define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)

static int fast_convergence __read_mostly = 1;
static int beta __read_mostly = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
static int initial_ssthresh __read_mostly;
static int bic_scale __read_mostly = 41;
static int tcp_friendliness __read_mostly = 1;

static int hystart __read_mostly = 1;
static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
static int hystart_low_window __read_mostly = 16;
static int hystart_ack_delta __read_mostly = 2;

static u32 cube_rtt_scale __read_mostly;
static u32 beta_scale __read_mostly;
static u64 cube_factor __read_mostly;

/* Note parameters that are used for precomputing scale factors are read-only */
module_param(fast_convergence, int, 0644);
MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "beta for multiplicative increase");
module_param(initial_ssthresh, int, 0644);
MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
module_param(bic_scale, int, 0444);
MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
module_param(tcp_friendliness, int, 0644);
MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
module_param(hystart, int, 0644);
MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
module_param(hystart_detect, int, 0644);
MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
		 " 1: packet-train 2: delay 3: both packet-train and delay");
module_param(hystart_low_window, int, 0644);
MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
module_param(hystart_ack_delta, int, 0644);
MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");

/* BIC TCP Parameters */
struct bictcp {
	u32	cnt;		/* increase cwnd by 1 after ACKs */
	u32	last_max_cwnd;	/* last maximum snd_cwnd */
	u32	loss_cwnd;	/* congestion window at last loss */
	u32	last_cwnd;	/* the last snd_cwnd */
	u32	last_time;	/* time when updated last_cwnd */
	u32	bic_origin_point;/* origin point of bic function */
	u32	bic_K;		/* time to origin point
				   from the beginning of the current epoch */
	u32	delay_min;	/* min delay (msec << 3) */
	u32	epoch_start;	/* beginning of an epoch */
	u32	ack_cnt;	/* number of acks */
	u32	tcp_cwnd;	/* estimated tcp cwnd */
	u16	unused;
	u8	sample_cnt;	/* number of samples to decide curr_rtt */
	u8	found;		/* the exit point is found? */
	u32	round_start;	/* beginning of each round */
	u32	end_seq;	/* end_seq of the round */
	u32	last_ack;	/* last time when the ACK spacing is close */
	u32	curr_rtt;	/* the minimum rtt of current round */
};

static inline void bictcp_reset(struct bictcp *ca)
{
	ca->cnt = 0;
	ca->last_max_cwnd = 0;
	ca->last_cwnd = 0;
	ca->last_time = 0;
	ca->bic_origin_point = 0;
	ca->bic_K = 0;
	ca->delay_min = 0;
	ca->epoch_start = 0;
	ca->ack_cnt = 0;
	ca->tcp_cwnd = 0;
	ca->found = 0;
}

static inline u32 bictcp_clock(void)
{
#if HZ < 1000
	return ktime_to_ms(ktime_get_real());
#else
	return jiffies_to_msecs(jiffies);
#endif
}

static inline void bictcp_hystart_reset(struct sock *sk)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	ca->round_start = ca->last_ack = bictcp_clock();
	ca->end_seq = tp->snd_nxt;
	ca->curr_rtt = 0;
	ca->sample_cnt = 0;
}

static void bictcp_init(struct sock *sk)
{
	struct bictcp *ca = inet_csk_ca(sk);

	bictcp_reset(ca);
	ca->loss_cwnd = 0;

	if (hystart)
		bictcp_hystart_reset(sk);

	if (!hystart && initial_ssthresh)
		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
}

static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{
	if (event == CA_EVENT_TX_START) {
		struct bictcp *ca = inet_csk_ca(sk);
		u32 now = tcp_time_stamp;
		s32 delta;

		delta = now - tcp_sk(sk)->lsndtime;

		/* We were application limited (idle) for a while.
		 * Shift epoch_start to keep cwnd growth to cubic curve.
		 */
		if (ca->epoch_start && delta > 0) {
			ca->epoch_start += delta;
			if (after(ca->epoch_start, now))
				ca->epoch_start = now;
		}
		return;
	}
}

/* calculate the cubic root of x using a table lookup followed by one
 * Newton-Raphson iteration.
 * Avg err ~= 0.195%
 */
static u32 cubic_root(u64 a)
{
	u32 x, b, shift;
	/*
	 * cbrt(x) MSB values for x MSB values in [0..63].
	 * Precomputed then refined by hand - Willy Tarreau
	 *
	 * For x in [0..63],
	 *   v = cbrt(x << 18) - 1
	 *   cbrt(x) = (v[x] + 10) >> 6
	 */
	static const u8 v[] = {
		/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
		/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
		/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
		/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
		/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
		/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
		/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
		/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
	};

	b = fls64(a);
	if (b < 7) {
		/* a in [0..63] */
		return ((u32)v[(u32)a] + 35) >> 6;
	}

	b = ((b * 84) >> 8) - 1;
	shift = (a >> (b * 3));

	x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;

	/*
	 * Newton-Raphson iteration
	 *                         2
	 * x    = ( 2 * x  +  a / x  ) / 3
	 *  k+1          k         k
	 */
	x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
	x = ((x * 341) >> 10);
	return x;
}

/*
 * Compute congestion window to use.
 */
static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
{
	u32 delta, bic_target, max_cnt;
	u64 offs, t;

	ca->ack_cnt += acked;	/* count the number of ACKed packets */

	if (ca->last_cwnd == cwnd &&
	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
		return;

	/* The CUBIC function can update ca->cnt at most once per jiffy.
	 * On all cwnd reduction events, ca->epoch_start is set to 0,
	 * which will force a recalculation of ca->cnt.
	 */
	if (ca->epoch_start && tcp_time_stamp == ca->last_time)
		goto tcp_friendliness;

	ca->last_cwnd = cwnd;
	ca->last_time = tcp_time_stamp;

	if (ca->epoch_start == 0) {
		ca->epoch_start = tcp_time_stamp;	/* record beginning */
		ca->ack_cnt = acked;			/* start counting */
		ca->tcp_cwnd = cwnd;			/* syn with cubic */

		if (ca->last_max_cwnd <= cwnd) {
			ca->bic_K = 0;
			ca->bic_origin_point = cwnd;
		} else {
			/* Compute new K based on
			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
			 */
			ca->bic_K = cubic_root(cube_factor
					       * (ca->last_max_cwnd - cwnd));
			ca->bic_origin_point = ca->last_max_cwnd;
		}
	}

	/* cubic function - calc*/
	/* calculate c * time^3 / rtt,
	 *  while considering overflow in calculation of time^3
	 * (so time^3 is done by using 64 bit)
	 * and without the support of division of 64bit numbers
	 * (so all divisions are done by using 32 bit)
	 *  also NOTE the unit of those veriables
	 *	  time  = (t - K) / 2^bictcp_HZ
	 *	  c = bic_scale >> 10
	 * rtt  = (srtt >> 3) / HZ
	 * !!! The following code does not have overflow problems,
	 * if the cwnd < 1 million packets !!!
	 */

	t = (s32)(tcp_time_stamp - ca->epoch_start);
	t += msecs_to_jiffies(ca->delay_min >> 3);
	/* change the unit from HZ to bictcp_HZ */
	t <<= BICTCP_HZ;
	do_div(t, HZ);

	if (t < ca->bic_K)		/* t - K */
		offs = ca->bic_K - t;
	else
		offs = t - ca->bic_K;

	/* c/rtt * (t-K)^3 */
	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
	if (t < ca->bic_K)                            /* below origin*/
		bic_target = ca->bic_origin_point - delta;
	else                                          /* above origin*/
		bic_target = ca->bic_origin_point + delta;

	/* cubic function - calc bictcp_cnt*/
	if (bic_target > cwnd) {
		ca->cnt = cwnd / (bic_target - cwnd);
	} else {
		ca->cnt = 100 * cwnd;              /* very small increment*/
	}

	/*
	 * The initial growth of cubic function may be too conservative
	 * when the available bandwidth is still unknown.
	 */
	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
		ca->cnt = 20;	/* increase cwnd 5% per RTT */

tcp_friendliness:
	/* TCP Friendly */
	if (tcp_friendliness) {
		u32 scale = beta_scale;

		delta = (cwnd * scale) >> 3;
		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
			ca->ack_cnt -= delta;
			ca->tcp_cwnd++;
		}

		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
			delta = ca->tcp_cwnd - cwnd;
			max_cnt = cwnd / delta;
			if (ca->cnt > max_cnt)
				ca->cnt = max_cnt;
		}
	}

	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
	 */
	ca->cnt = max(ca->cnt, 2U);
}

static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	if (!tcp_is_cwnd_limited(sk))
		return;

	if (tcp_in_slow_start(tp)) {
		if (hystart && after(ack, ca->end_seq))
			bictcp_hystart_reset(sk);
		acked = tcp_slow_start(tp, acked);
		if (!acked)
			return;
	}
	bictcp_update(ca, tp->snd_cwnd, acked);
	tcp_cong_avoid_ai(tp, ca->cnt, acked);
}

static u32 bictcp_recalc_ssthresh(struct sock *sk)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	ca->epoch_start = 0;	/* end of epoch */

	/* Wmax and fast convergence */
	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
			/ (2 * BICTCP_BETA_SCALE);
	else
		ca->last_max_cwnd = tp->snd_cwnd;

	ca->loss_cwnd = tp->snd_cwnd;

	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
}

static u32 bictcp_undo_cwnd(struct sock *sk)
{
	struct bictcp *ca = inet_csk_ca(sk);

	return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
}

static void bictcp_state(struct sock *sk, u8 new_state)
{
	if (new_state == TCP_CA_Loss) {
		bictcp_reset(inet_csk_ca(sk));
		bictcp_hystart_reset(sk);
	}
}

static void hystart_update(struct sock *sk, u32 delay)
{
	struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);

	if (ca->found & hystart_detect)
		return;

	if (hystart_detect & HYSTART_ACK_TRAIN) {
		u32 now = bictcp_clock();

		/* first detection parameter - ack-train detection */
		if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
			ca->last_ack = now;
			if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
				ca->found |= HYSTART_ACK_TRAIN;
				__NET_INC_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTTRAINDETECT);
				__NET_ADD_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTTRAINCWND,
						tp->snd_cwnd);
				tp->snd_ssthresh = tp->snd_cwnd;
			}
		}
	}

	if (hystart_detect & HYSTART_DELAY) {
		/* obtain the minimum delay of more than sampling packets */
		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
			if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
				ca->curr_rtt = delay;

			ca->sample_cnt++;
		} else {
			if (ca->curr_rtt > ca->delay_min +
			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
				ca->found |= HYSTART_DELAY;
				__NET_INC_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTDELAYDETECT);
				__NET_ADD_STATS(sock_net(sk),
						LINUX_MIB_TCPHYSTARTDELAYCWND,
						tp->snd_cwnd);
				tp->snd_ssthresh = tp->snd_cwnd;
			}
		}
	}
}

/* Track delayed acknowledgment ratio using sliding window
 * ratio = (15*ratio + sample) / 16
 */
static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
{
	const struct tcp_sock *tp = tcp_sk(sk);
	struct bictcp *ca = inet_csk_ca(sk);
	u32 delay;

	/* Some calls are for duplicates without timetamps */
	if (rtt_us < 0)
		return;

	/* Discard delay samples right after fast recovery */
	if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
		return;

	delay = (rtt_us << 3) / USEC_PER_MSEC;
	if (delay == 0)
		delay = 1;

	/* first time call or link delay decreases */
	if (ca->delay_min == 0 || ca->delay_min > delay)
		ca->delay_min = delay;

	/* hystart triggers when cwnd is larger than some threshold */
	if (hystart && tcp_in_slow_start(tp) &&
	    tp->snd_cwnd >= hystart_low_window)
		hystart_update(sk, delay);
}

static struct tcp_congestion_ops cubictcp __read_mostly = {
	.init		= bictcp_init,
	.ssthresh	= bictcp_recalc_ssthresh,
	.cong_avoid	= bictcp_cong_avoid,
	.set_state	= bictcp_state,
	.undo_cwnd	= bictcp_undo_cwnd,
	.cwnd_event	= bictcp_cwnd_event,
	.pkts_acked     = bictcp_acked,
	.owner		= THIS_MODULE,
	.name		= "cubic",
};

static int __init cubictcp_register(void)
{
	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);

	/* Precompute a bunch of the scaling factors that are used per-packet
	 * based on SRTT of 100ms
	 */

	beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
		/ (BICTCP_BETA_SCALE - beta);

	cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */

	/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
	 *  so K = cubic_root( (wmax-cwnd)*rtt/c )
	 * the unit of K is bictcp_HZ=2^10, not HZ
	 *
	 *  c = bic_scale >> 10
	 *  rtt = 100ms
	 *
	 * the following code has been designed and tested for
	 * cwnd < 1 million packets
	 * RTT < 100 seconds
	 * HZ < 1,000,00  (corresponding to 10 nano-second)
	 */

	/* 1/c * 2^2*bictcp_HZ * srtt */
	cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */

	/* divide by bic_scale and by constant Srtt (100ms) */
	do_div(cube_factor, bic_scale * 10);

	return tcp_register_congestion_control(&cubictcp);
}

static void __exit cubictcp_unregister(void)
{
	tcp_unregister_congestion_control(&cubictcp);
}

module_init(cubictcp_register);
module_exit(cubictcp_unregister);

MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("CUBIC TCP");
MODULE_VERSION("2.3");