1/*
2 * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
3 * Home page:
4 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
5 * This is from the implementation of CUBIC TCP in
6 * Sangtae Ha, Injong Rhee and Lisong Xu,
7 *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
8 *  in ACM SIGOPS Operating System Review, July 2008.
9 * Available from:
10 *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
11 *
12 * CUBIC integrates a new slow start algorithm, called HyStart.
13 * The details of HyStart are presented in
14 *  Sangtae Ha and Injong Rhee,
15 *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
16 * Available from:
17 *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
18 *
19 * All testing results are available from:
20 * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
21 *
22 * Unless CUBIC is enabled and congestion window is large
23 * this behaves the same as the original Reno.
24 */
25
26#include <linux/mm.h>
27#include <linux/module.h>
28#include <linux/math64.h>
29#include <net/tcp.h>
30
31#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
32					 * max_cwnd = snd_cwnd * beta
33					 */
34#define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
35
36/* Two methods of hybrid slow start */
37#define HYSTART_ACK_TRAIN	0x1
38#define HYSTART_DELAY		0x2
39
40/* Number of delay samples for detecting the increase of delay */
41#define HYSTART_MIN_SAMPLES	8
42#define HYSTART_DELAY_MIN	(4U<<3)
43#define HYSTART_DELAY_MAX	(16U<<3)
44#define HYSTART_DELAY_THRESH(x)	clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
45
46static int fast_convergence __read_mostly = 1;
47static int beta __read_mostly = 717;	/* = 717/1024 (BICTCP_BETA_SCALE) */
48static int initial_ssthresh __read_mostly;
49static int bic_scale __read_mostly = 41;
50static int tcp_friendliness __read_mostly = 1;
51
52static int hystart __read_mostly = 1;
53static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
54static int hystart_low_window __read_mostly = 16;
55static int hystart_ack_delta __read_mostly = 2;
56
57static u32 cube_rtt_scale __read_mostly;
58static u32 beta_scale __read_mostly;
59static u64 cube_factor __read_mostly;
60
61/* Note parameters that are used for precomputing scale factors are read-only */
62module_param(fast_convergence, int, 0644);
63MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
64module_param(beta, int, 0644);
65MODULE_PARM_DESC(beta, "beta for multiplicative increase");
66module_param(initial_ssthresh, int, 0644);
67MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
68module_param(bic_scale, int, 0444);
69MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
70module_param(tcp_friendliness, int, 0644);
71MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
72module_param(hystart, int, 0644);
73MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
74module_param(hystart_detect, int, 0644);
75MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
76		 " 1: packet-train 2: delay 3: both packet-train and delay");
77module_param(hystart_low_window, int, 0644);
78MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
79module_param(hystart_ack_delta, int, 0644);
80MODULE_PARM_DESC(hystart_ack_delta, "spacing between ack's indicating train (msecs)");
81
82/* BIC TCP Parameters */
83struct bictcp {
84	u32	cnt;		/* increase cwnd by 1 after ACKs */
85	u32	last_max_cwnd;	/* last maximum snd_cwnd */
86	u32	loss_cwnd;	/* congestion window at last loss */
87	u32	last_cwnd;	/* the last snd_cwnd */
88	u32	last_time;	/* time when updated last_cwnd */
89	u32	bic_origin_point;/* origin point of bic function */
90	u32	bic_K;		/* time to origin point
91				   from the beginning of the current epoch */
92	u32	delay_min;	/* min delay (msec << 3) */
93	u32	epoch_start;	/* beginning of an epoch */
94	u32	ack_cnt;	/* number of acks */
95	u32	tcp_cwnd;	/* estimated tcp cwnd */
96	u16	unused;
97	u8	sample_cnt;	/* number of samples to decide curr_rtt */
98	u8	found;		/* the exit point is found? */
99	u32	round_start;	/* beginning of each round */
100	u32	end_seq;	/* end_seq of the round */
101	u32	last_ack;	/* last time when the ACK spacing is close */
102	u32	curr_rtt;	/* the minimum rtt of current round */
103};
104
105static inline void bictcp_reset(struct bictcp *ca)
106{
107	ca->cnt = 0;
108	ca->last_max_cwnd = 0;
109	ca->last_cwnd = 0;
110	ca->last_time = 0;
111	ca->bic_origin_point = 0;
112	ca->bic_K = 0;
113	ca->delay_min = 0;
114	ca->epoch_start = 0;
115	ca->ack_cnt = 0;
116	ca->tcp_cwnd = 0;
117	ca->found = 0;
118}
119
120static inline u32 bictcp_clock(void)
121{
122#if HZ < 1000
123	return ktime_to_ms(ktime_get_real());
124#else
125	return jiffies_to_msecs(jiffies);
126#endif
127}
128
129static inline void bictcp_hystart_reset(struct sock *sk)
130{
131	struct tcp_sock *tp = tcp_sk(sk);
132	struct bictcp *ca = inet_csk_ca(sk);
133
134	ca->round_start = ca->last_ack = bictcp_clock();
135	ca->end_seq = tp->snd_nxt;
136	ca->curr_rtt = 0;
137	ca->sample_cnt = 0;
138}
139
140static void bictcp_init(struct sock *sk)
141{
142	struct bictcp *ca = inet_csk_ca(sk);
143
144	bictcp_reset(ca);
145	ca->loss_cwnd = 0;
146
147	if (hystart)
148		bictcp_hystart_reset(sk);
149
150	if (!hystart && initial_ssthresh)
151		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
152}
153
154static void bictcp_cwnd_event(struct sock *sk, enum tcp_ca_event event)
155{
156	if (event == CA_EVENT_TX_START) {
157		struct bictcp *ca = inet_csk_ca(sk);
158		u32 now = tcp_time_stamp;
159		s32 delta;
160
161		delta = now - tcp_sk(sk)->lsndtime;
162
163		/* We were application limited (idle) for a while.
164		 * Shift epoch_start to keep cwnd growth to cubic curve.
165		 */
166		if (ca->epoch_start && delta > 0) {
167			ca->epoch_start += delta;
168			if (after(ca->epoch_start, now))
169				ca->epoch_start = now;
170		}
171		return;
172	}
173}
174
175/* calculate the cubic root of x using a table lookup followed by one
176 * Newton-Raphson iteration.
177 * Avg err ~= 0.195%
178 */
179static u32 cubic_root(u64 a)
180{
181	u32 x, b, shift;
182	/*
183	 * cbrt(x) MSB values for x MSB values in [0..63].
184	 * Precomputed then refined by hand - Willy Tarreau
185	 *
186	 * For x in [0..63],
187	 *   v = cbrt(x << 18) - 1
188	 *   cbrt(x) = (v[x] + 10) >> 6
189	 */
190	static const u8 v[] = {
191		/* 0x00 */    0,   54,   54,   54,  118,  118,  118,  118,
192		/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
193		/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
194		/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
195		/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
196		/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
197		/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
198		/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
199	};
200
201	b = fls64(a);
202	if (b < 7) {
203		/* a in [0..63] */
204		return ((u32)v[(u32)a] + 35) >> 6;
205	}
206
207	b = ((b * 84) >> 8) - 1;
208	shift = (a >> (b * 3));
209
210	x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
211
212	/*
213	 * Newton-Raphson iteration
214	 *                         2
215	 * x    = ( 2 * x  +  a / x  ) / 3
216	 *  k+1          k         k
217	 */
218	x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
219	x = ((x * 341) >> 10);
220	return x;
221}
222
223/*
224 * Compute congestion window to use.
225 */
226static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
227{
228	u32 delta, bic_target, max_cnt;
229	u64 offs, t;
230
231	ca->ack_cnt += acked;	/* count the number of ACKed packets */
232
233	if (ca->last_cwnd == cwnd &&
234	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
235		return;
236
237	/* The CUBIC function can update ca->cnt at most once per jiffy.
238	 * On all cwnd reduction events, ca->epoch_start is set to 0,
239	 * which will force a recalculation of ca->cnt.
240	 */
241	if (ca->epoch_start && tcp_time_stamp == ca->last_time)
242		goto tcp_friendliness;
243
244	ca->last_cwnd = cwnd;
245	ca->last_time = tcp_time_stamp;
246
247	if (ca->epoch_start == 0) {
248		ca->epoch_start = tcp_time_stamp;	/* record beginning */
249		ca->ack_cnt = acked;			/* start counting */
250		ca->tcp_cwnd = cwnd;			/* syn with cubic */
251
252		if (ca->last_max_cwnd <= cwnd) {
253			ca->bic_K = 0;
254			ca->bic_origin_point = cwnd;
255		} else {
256			/* Compute new K based on
257			 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
258			 */
259			ca->bic_K = cubic_root(cube_factor
260					       * (ca->last_max_cwnd - cwnd));
261			ca->bic_origin_point = ca->last_max_cwnd;
262		}
263	}
264
265	/* cubic function - calc*/
266	/* calculate c * time^3 / rtt,
267	 *  while considering overflow in calculation of time^3
268	 * (so time^3 is done by using 64 bit)
269	 * and without the support of division of 64bit numbers
270	 * (so all divisions are done by using 32 bit)
271	 *  also NOTE the unit of those veriables
272	 *	  time  = (t - K) / 2^bictcp_HZ
273	 *	  c = bic_scale >> 10
274	 * rtt  = (srtt >> 3) / HZ
275	 * !!! The following code does not have overflow problems,
276	 * if the cwnd < 1 million packets !!!
277	 */
278
279	t = (s32)(tcp_time_stamp - ca->epoch_start);
280	t += msecs_to_jiffies(ca->delay_min >> 3);
281	/* change the unit from HZ to bictcp_HZ */
282	t <<= BICTCP_HZ;
283	do_div(t, HZ);
284
285	if (t < ca->bic_K)		/* t - K */
286		offs = ca->bic_K - t;
287	else
288		offs = t - ca->bic_K;
289
290	/* c/rtt * (t-K)^3 */
291	delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
292	if (t < ca->bic_K)                            /* below origin*/
293		bic_target = ca->bic_origin_point - delta;
294	else                                          /* above origin*/
295		bic_target = ca->bic_origin_point + delta;
296
297	/* cubic function - calc bictcp_cnt*/
298	if (bic_target > cwnd) {
299		ca->cnt = cwnd / (bic_target - cwnd);
300	} else {
301		ca->cnt = 100 * cwnd;              /* very small increment*/
302	}
303
304	/*
305	 * The initial growth of cubic function may be too conservative
306	 * when the available bandwidth is still unknown.
307	 */
308	if (ca->last_max_cwnd == 0 && ca->cnt > 20)
309		ca->cnt = 20;	/* increase cwnd 5% per RTT */
310
311tcp_friendliness:
312	/* TCP Friendly */
313	if (tcp_friendliness) {
314		u32 scale = beta_scale;
315
316		delta = (cwnd * scale) >> 3;
317		while (ca->ack_cnt > delta) {		/* update tcp cwnd */
318			ca->ack_cnt -= delta;
319			ca->tcp_cwnd++;
320		}
321
322		if (ca->tcp_cwnd > cwnd) {	/* if bic is slower than tcp */
323			delta = ca->tcp_cwnd - cwnd;
324			max_cnt = cwnd / delta;
325			if (ca->cnt > max_cnt)
326				ca->cnt = max_cnt;
327		}
328	}
329
330	/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
331	 * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
332	 */
333	ca->cnt = max(ca->cnt, 2U);
334}
335
336static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
337{
338	struct tcp_sock *tp = tcp_sk(sk);
339	struct bictcp *ca = inet_csk_ca(sk);
340
341	if (!tcp_is_cwnd_limited(sk))
342		return;
343
344	if (tcp_in_slow_start(tp)) {
345		if (hystart && after(ack, ca->end_seq))
346			bictcp_hystart_reset(sk);
347		acked = tcp_slow_start(tp, acked);
348		if (!acked)
349			return;
350	}
351	bictcp_update(ca, tp->snd_cwnd, acked);
352	tcp_cong_avoid_ai(tp, ca->cnt, acked);
353}
354
355static u32 bictcp_recalc_ssthresh(struct sock *sk)
356{
357	const struct tcp_sock *tp = tcp_sk(sk);
358	struct bictcp *ca = inet_csk_ca(sk);
359
360	ca->epoch_start = 0;	/* end of epoch */
361
362	/* Wmax and fast convergence */
363	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
364		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
365			/ (2 * BICTCP_BETA_SCALE);
366	else
367		ca->last_max_cwnd = tp->snd_cwnd;
368
369	ca->loss_cwnd = tp->snd_cwnd;
370
371	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
372}
373
374static u32 bictcp_undo_cwnd(struct sock *sk)
375{
376	struct bictcp *ca = inet_csk_ca(sk);
377
378	return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
379}
380
381static void bictcp_state(struct sock *sk, u8 new_state)
382{
383	if (new_state == TCP_CA_Loss) {
384		bictcp_reset(inet_csk_ca(sk));
385		bictcp_hystart_reset(sk);
386	}
387}
388
389static void hystart_update(struct sock *sk, u32 delay)
390{
391	struct tcp_sock *tp = tcp_sk(sk);
392	struct bictcp *ca = inet_csk_ca(sk);
393
394	if (ca->found & hystart_detect)
395		return;
396
397	if (hystart_detect & HYSTART_ACK_TRAIN) {
398		u32 now = bictcp_clock();
399
400		/* first detection parameter - ack-train detection */
401		if ((s32)(now - ca->last_ack) <= hystart_ack_delta) {
402			ca->last_ack = now;
403			if ((s32)(now - ca->round_start) > ca->delay_min >> 4) {
404				ca->found |= HYSTART_ACK_TRAIN;
405				NET_INC_STATS_BH(sock_net(sk),
406						 LINUX_MIB_TCPHYSTARTTRAINDETECT);
407				NET_ADD_STATS_BH(sock_net(sk),
408						 LINUX_MIB_TCPHYSTARTTRAINCWND,
409						 tp->snd_cwnd);
410				tp->snd_ssthresh = tp->snd_cwnd;
411			}
412		}
413	}
414
415	if (hystart_detect & HYSTART_DELAY) {
416		/* obtain the minimum delay of more than sampling packets */
417		if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
418			if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
419				ca->curr_rtt = delay;
420
421			ca->sample_cnt++;
422		} else {
423			if (ca->curr_rtt > ca->delay_min +
424			    HYSTART_DELAY_THRESH(ca->delay_min >> 3)) {
425				ca->found |= HYSTART_DELAY;
426				NET_INC_STATS_BH(sock_net(sk),
427						 LINUX_MIB_TCPHYSTARTDELAYDETECT);
428				NET_ADD_STATS_BH(sock_net(sk),
429						 LINUX_MIB_TCPHYSTARTDELAYCWND,
430						 tp->snd_cwnd);
431				tp->snd_ssthresh = tp->snd_cwnd;
432			}
433		}
434	}
435}
436
437/* Track delayed acknowledgment ratio using sliding window
438 * ratio = (15*ratio + sample) / 16
439 */
440static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
441{
442	const struct tcp_sock *tp = tcp_sk(sk);
443	struct bictcp *ca = inet_csk_ca(sk);
444	u32 delay;
445
446	/* Some calls are for duplicates without timetamps */
447	if (rtt_us < 0)
448		return;
449
450	/* Discard delay samples right after fast recovery */
451	if (ca->epoch_start && (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
452		return;
453
454	delay = (rtt_us << 3) / USEC_PER_MSEC;
455	if (delay == 0)
456		delay = 1;
457
458	/* first time call or link delay decreases */
459	if (ca->delay_min == 0 || ca->delay_min > delay)
460		ca->delay_min = delay;
461
462	/* hystart triggers when cwnd is larger than some threshold */
463	if (hystart && tcp_in_slow_start(tp) &&
464	    tp->snd_cwnd >= hystart_low_window)
465		hystart_update(sk, delay);
466}
467
468static struct tcp_congestion_ops cubictcp __read_mostly = {
469	.init		= bictcp_init,
470	.ssthresh	= bictcp_recalc_ssthresh,
471	.cong_avoid	= bictcp_cong_avoid,
472	.set_state	= bictcp_state,
473	.undo_cwnd	= bictcp_undo_cwnd,
474	.cwnd_event	= bictcp_cwnd_event,
475	.pkts_acked     = bictcp_acked,
476	.owner		= THIS_MODULE,
477	.name		= "cubic",
478};
479
480static int __init cubictcp_register(void)
481{
482	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
483
484	/* Precompute a bunch of the scaling factors that are used per-packet
485	 * based on SRTT of 100ms
486	 */
487
488	beta_scale = 8*(BICTCP_BETA_SCALE+beta) / 3
489		/ (BICTCP_BETA_SCALE - beta);
490
491	cube_rtt_scale = (bic_scale * 10);	/* 1024*c/rtt */
492
493	/* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
494	 *  so K = cubic_root( (wmax-cwnd)*rtt/c )
495	 * the unit of K is bictcp_HZ=2^10, not HZ
496	 *
497	 *  c = bic_scale >> 10
498	 *  rtt = 100ms
499	 *
500	 * the following code has been designed and tested for
501	 * cwnd < 1 million packets
502	 * RTT < 100 seconds
503	 * HZ < 1,000,00  (corresponding to 10 nano-second)
504	 */
505
506	/* 1/c * 2^2*bictcp_HZ * srtt */
507	cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
508
509	/* divide by bic_scale and by constant Srtt (100ms) */
510	do_div(cube_factor, bic_scale * 10);
511
512	return tcp_register_congestion_control(&cubictcp);
513}
514
515static void __exit cubictcp_unregister(void)
516{
517	tcp_unregister_congestion_control(&cubictcp);
518}
519
520module_init(cubictcp_register);
521module_exit(cubictcp_unregister);
522
523MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
524MODULE_LICENSE("GPL");
525MODULE_DESCRIPTION("CUBIC TCP");
526MODULE_VERSION("2.3");
527