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 (2U<<3)
43 #define HYSTART_DELAY_MAX (16U<<3)
44 #define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
45
46 static int fast_convergence __read_mostly = 1;
47 static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */
48 static int initial_ssthresh __read_mostly;
49 static int bic_scale __read_mostly = 41;
50 static int tcp_friendliness __read_mostly = 1;
51
52 static int hystart __read_mostly = 1;
53 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
54 static int hystart_low_window __read_mostly = 16;
55
56 static u32 cube_rtt_scale __read_mostly;
57 static u32 beta_scale __read_mostly;
58 static u64 cube_factor __read_mostly;
59
60 /* Note parameters that are used for precomputing scale factors are read-only */
61 module_param(fast_convergence, int, 0644);
62 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
63 module_param(beta, int, 0644);
64 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
65 module_param(initial_ssthresh, int, 0644);
66 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
67 module_param(bic_scale, int, 0444);
68 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
69 module_param(tcp_friendliness, int, 0644);
70 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
71 module_param(hystart, int, 0644);
72 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
73 module_param(hystart_detect, int, 0644);
74 MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
75 " 1: packet-train 2: delay 3: both packet-train and delay");
76 module_param(hystart_low_window, int, 0644);
77 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
78
79 /* BIC TCP Parameters */
80 struct bictcp {
81 u32 cnt; /* increase cwnd by 1 after ACKs */
82 u32 last_max_cwnd; /* last maximum snd_cwnd */
83 u32 loss_cwnd; /* congestion window at last loss */
84 u32 last_cwnd; /* the last snd_cwnd */
85 u32 last_time; /* time when updated last_cwnd */
86 u32 bic_origin_point;/* origin point of bic function */
87 u32 bic_K; /* time to origin point from the beginning of the current epoch */
88 u32 delay_min; /* min delay */
89 u32 epoch_start; /* beginning of an epoch */
90 u32 ack_cnt; /* number of acks */
91 u32 tcp_cwnd; /* estimated tcp cwnd */
92 #define ACK_RATIO_SHIFT 4
93 u16 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */
94 u8 sample_cnt; /* number of samples to decide curr_rtt */
95 u8 found; /* the exit point is found? */
96 u32 round_start; /* beginning of each round */
97 u32 end_seq; /* end_seq of the round */
98 u32 last_jiffies; /* last time when the ACK spacing is close */
99 u32 curr_rtt; /* the minimum rtt of current round */
100 };
101
bictcp_reset(struct bictcp * ca)102 static inline void bictcp_reset(struct bictcp *ca)
103 {
104 ca->cnt = 0;
105 ca->last_max_cwnd = 0;
106 ca->loss_cwnd = 0;
107 ca->last_cwnd = 0;
108 ca->last_time = 0;
109 ca->bic_origin_point = 0;
110 ca->bic_K = 0;
111 ca->delay_min = 0;
112 ca->epoch_start = 0;
113 ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
114 ca->ack_cnt = 0;
115 ca->tcp_cwnd = 0;
116 ca->found = 0;
117 }
118
bictcp_hystart_reset(struct sock * sk)119 static inline void bictcp_hystart_reset(struct sock *sk)
120 {
121 struct tcp_sock *tp = tcp_sk(sk);
122 struct bictcp *ca = inet_csk_ca(sk);
123
124 ca->round_start = ca->last_jiffies = jiffies;
125 ca->end_seq = tp->snd_nxt;
126 ca->curr_rtt = 0;
127 ca->sample_cnt = 0;
128 }
129
bictcp_init(struct sock * sk)130 static void bictcp_init(struct sock *sk)
131 {
132 bictcp_reset(inet_csk_ca(sk));
133
134 if (hystart)
135 bictcp_hystart_reset(sk);
136
137 if (!hystart && initial_ssthresh)
138 tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
139 }
140
141 /* calculate the cubic root of x using a table lookup followed by one
142 * Newton-Raphson iteration.
143 * Avg err ~= 0.195%
144 */
cubic_root(u64 a)145 static u32 cubic_root(u64 a)
146 {
147 u32 x, b, shift;
148 /*
149 * cbrt(x) MSB values for x MSB values in [0..63].
150 * Precomputed then refined by hand - Willy Tarreau
151 *
152 * For x in [0..63],
153 * v = cbrt(x << 18) - 1
154 * cbrt(x) = (v[x] + 10) >> 6
155 */
156 static const u8 v[] = {
157 /* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118,
158 /* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156,
159 /* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179,
160 /* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199,
161 /* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215,
162 /* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229,
163 /* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242,
164 /* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254,
165 };
166
167 b = fls64(a);
168 if (b < 7) {
169 /* a in [0..63] */
170 return ((u32)v[(u32)a] + 35) >> 6;
171 }
172
173 b = ((b * 84) >> 8) - 1;
174 shift = (a >> (b * 3));
175
176 x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
177
178 /*
179 * Newton-Raphson iteration
180 * 2
181 * x = ( 2 * x + a / x ) / 3
182 * k+1 k k
183 */
184 x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
185 x = ((x * 341) >> 10);
186 return x;
187 }
188
189 /*
190 * Compute congestion window to use.
191 */
bictcp_update(struct bictcp * ca,u32 cwnd)192 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
193 {
194 u64 offs;
195 u32 delta, t, bic_target, max_cnt;
196
197 ca->ack_cnt++; /* count the number of ACKs */
198
199 if (ca->last_cwnd == cwnd &&
200 (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
201 return;
202
203 ca->last_cwnd = cwnd;
204 ca->last_time = tcp_time_stamp;
205
206 if (ca->epoch_start == 0) {
207 ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */
208 ca->ack_cnt = 1; /* start counting */
209 ca->tcp_cwnd = cwnd; /* syn with cubic */
210
211 if (ca->last_max_cwnd <= cwnd) {
212 ca->bic_K = 0;
213 ca->bic_origin_point = cwnd;
214 } else {
215 /* Compute new K based on
216 * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
217 */
218 ca->bic_K = cubic_root(cube_factor
219 * (ca->last_max_cwnd - cwnd));
220 ca->bic_origin_point = ca->last_max_cwnd;
221 }
222 }
223
224 /* cubic function - calc*/
225 /* calculate c * time^3 / rtt,
226 * while considering overflow in calculation of time^3
227 * (so time^3 is done by using 64 bit)
228 * and without the support of division of 64bit numbers
229 * (so all divisions are done by using 32 bit)
230 * also NOTE the unit of those veriables
231 * time = (t - K) / 2^bictcp_HZ
232 * c = bic_scale >> 10
233 * rtt = (srtt >> 3) / HZ
234 * !!! The following code does not have overflow problems,
235 * if the cwnd < 1 million packets !!!
236 */
237
238 /* change the unit from HZ to bictcp_HZ */
239 t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
240 << BICTCP_HZ) / HZ;
241
242 if (t < ca->bic_K) /* t - K */
243 offs = ca->bic_K - t;
244 else
245 offs = t - ca->bic_K;
246
247 /* c/rtt * (t-K)^3 */
248 delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
249 if (t < ca->bic_K) /* below origin*/
250 bic_target = ca->bic_origin_point - delta;
251 else /* above origin*/
252 bic_target = ca->bic_origin_point + delta;
253
254 /* cubic function - calc bictcp_cnt*/
255 if (bic_target > cwnd) {
256 ca->cnt = cwnd / (bic_target - cwnd);
257 } else {
258 ca->cnt = 100 * cwnd; /* very small increment*/
259 }
260
261 /* TCP Friendly */
262 if (tcp_friendliness) {
263 u32 scale = beta_scale;
264 delta = (cwnd * scale) >> 3;
265 while (ca->ack_cnt > delta) { /* update tcp cwnd */
266 ca->ack_cnt -= delta;
267 ca->tcp_cwnd++;
268 }
269
270 if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */
271 delta = ca->tcp_cwnd - cwnd;
272 max_cnt = cwnd / delta;
273 if (ca->cnt > max_cnt)
274 ca->cnt = max_cnt;
275 }
276 }
277
278 ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
279 if (ca->cnt == 0) /* cannot be zero */
280 ca->cnt = 1;
281 }
282
bictcp_cong_avoid(struct sock * sk,u32 ack,u32 in_flight)283 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
284 {
285 struct tcp_sock *tp = tcp_sk(sk);
286 struct bictcp *ca = inet_csk_ca(sk);
287
288 if (!tcp_is_cwnd_limited(sk, in_flight))
289 return;
290
291 if (tp->snd_cwnd <= tp->snd_ssthresh) {
292 if (hystart && after(ack, ca->end_seq))
293 bictcp_hystart_reset(sk);
294 tcp_slow_start(tp);
295 } else {
296 bictcp_update(ca, tp->snd_cwnd);
297
298 /* In dangerous area, increase slowly.
299 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
300 */
301 if (tp->snd_cwnd_cnt >= ca->cnt) {
302 if (tp->snd_cwnd < tp->snd_cwnd_clamp)
303 tp->snd_cwnd++;
304 tp->snd_cwnd_cnt = 0;
305 } else
306 tp->snd_cwnd_cnt++;
307 }
308
309 }
310
bictcp_recalc_ssthresh(struct sock * sk)311 static u32 bictcp_recalc_ssthresh(struct sock *sk)
312 {
313 const struct tcp_sock *tp = tcp_sk(sk);
314 struct bictcp *ca = inet_csk_ca(sk);
315
316 ca->epoch_start = 0; /* end of epoch */
317
318 /* Wmax and fast convergence */
319 if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
320 ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
321 / (2 * BICTCP_BETA_SCALE);
322 else
323 ca->last_max_cwnd = tp->snd_cwnd;
324
325 ca->loss_cwnd = tp->snd_cwnd;
326
327 return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
328 }
329
bictcp_undo_cwnd(struct sock * sk)330 static u32 bictcp_undo_cwnd(struct sock *sk)
331 {
332 struct bictcp *ca = inet_csk_ca(sk);
333
334 return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
335 }
336
bictcp_state(struct sock * sk,u8 new_state)337 static void bictcp_state(struct sock *sk, u8 new_state)
338 {
339 if (new_state == TCP_CA_Loss) {
340 bictcp_reset(inet_csk_ca(sk));
341 bictcp_hystart_reset(sk);
342 }
343 }
344
hystart_update(struct sock * sk,u32 delay)345 static void hystart_update(struct sock *sk, u32 delay)
346 {
347 struct tcp_sock *tp = tcp_sk(sk);
348 struct bictcp *ca = inet_csk_ca(sk);
349
350 if (!(ca->found & hystart_detect)) {
351 u32 curr_jiffies = jiffies;
352
353 /* first detection parameter - ack-train detection */
354 if (curr_jiffies - ca->last_jiffies <= msecs_to_jiffies(2)) {
355 ca->last_jiffies = curr_jiffies;
356 if (curr_jiffies - ca->round_start >= ca->delay_min>>4)
357 ca->found |= HYSTART_ACK_TRAIN;
358 }
359
360 /* obtain the minimum delay of more than sampling packets */
361 if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
362 if (ca->curr_rtt == 0 || ca->curr_rtt > delay)
363 ca->curr_rtt = delay;
364
365 ca->sample_cnt++;
366 } else {
367 if (ca->curr_rtt > ca->delay_min +
368 HYSTART_DELAY_THRESH(ca->delay_min>>4))
369 ca->found |= HYSTART_DELAY;
370 }
371 /*
372 * Either one of two conditions are met,
373 * we exit from slow start immediately.
374 */
375 if (ca->found & hystart_detect)
376 tp->snd_ssthresh = tp->snd_cwnd;
377 }
378 }
379
380 /* Track delayed acknowledgment ratio using sliding window
381 * ratio = (15*ratio + sample) / 16
382 */
bictcp_acked(struct sock * sk,u32 cnt,s32 rtt_us)383 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
384 {
385 const struct inet_connection_sock *icsk = inet_csk(sk);
386 const struct tcp_sock *tp = tcp_sk(sk);
387 struct bictcp *ca = inet_csk_ca(sk);
388 u32 delay;
389
390 if (icsk->icsk_ca_state == TCP_CA_Open) {
391 cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
392 ca->delayed_ack += cnt;
393 }
394
395 /* Some calls are for duplicates without timetamps */
396 if (rtt_us < 0)
397 return;
398
399 /* Discard delay samples right after fast recovery */
400 if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
401 return;
402
403 delay = usecs_to_jiffies(rtt_us) << 3;
404 if (delay == 0)
405 delay = 1;
406
407 /* first time call or link delay decreases */
408 if (ca->delay_min == 0 || ca->delay_min > delay)
409 ca->delay_min = delay;
410
411 /* hystart triggers when cwnd is larger than some threshold */
412 if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
413 tp->snd_cwnd >= hystart_low_window)
414 hystart_update(sk, delay);
415 }
416
417 static struct tcp_congestion_ops cubictcp = {
418 .init = bictcp_init,
419 .ssthresh = bictcp_recalc_ssthresh,
420 .cong_avoid = bictcp_cong_avoid,
421 .set_state = bictcp_state,
422 .undo_cwnd = bictcp_undo_cwnd,
423 .pkts_acked = bictcp_acked,
424 .owner = THIS_MODULE,
425 .name = "cubic",
426 };
427
cubictcp_register(void)428 static int __init cubictcp_register(void)
429 {
430 BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
431
432 /* Precompute a bunch of the scaling factors that are used per-packet
433 * based on SRTT of 100ms
434 */
435
436 beta_scale = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);
437
438 cube_rtt_scale = (bic_scale * 10); /* 1024*c/rtt */
439
440 /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
441 * so K = cubic_root( (wmax-cwnd)*rtt/c )
442 * the unit of K is bictcp_HZ=2^10, not HZ
443 *
444 * c = bic_scale >> 10
445 * rtt = 100ms
446 *
447 * the following code has been designed and tested for
448 * cwnd < 1 million packets
449 * RTT < 100 seconds
450 * HZ < 1,000,00 (corresponding to 10 nano-second)
451 */
452
453 /* 1/c * 2^2*bictcp_HZ * srtt */
454 cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
455
456 /* divide by bic_scale and by constant Srtt (100ms) */
457 do_div(cube_factor, bic_scale * 10);
458
459 return tcp_register_congestion_control(&cubictcp);
460 }
461
cubictcp_unregister(void)462 static void __exit cubictcp_unregister(void)
463 {
464 tcp_unregister_congestion_control(&cubictcp);
465 }
466
467 module_init(cubictcp_register);
468 module_exit(cubictcp_unregister);
469
470 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
471 MODULE_LICENSE("GPL");
472 MODULE_DESCRIPTION("CUBIC TCP");
473 MODULE_VERSION("2.3");
474