xref: /openbmc/linux/net/ipv4/tcp_bic.c (revision d5cb9783536a41df9f9cba5b0a1d78047ed787f7)
1 /*
2  * Binary Increase Congestion control for TCP
3  *
4  * This is from the implementation of BICTCP in
5  * Lison-Xu, Kahaled Harfoush, and Injong Rhee.
6  *  "Binary Increase Congestion Control for Fast, Long Distance
7  *  Networks" in InfoComm 2004
8  * Available from:
9  *  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp.pdf
10  *
11  * Unless BIC is enabled and congestion window is large
12  * this behaves the same as the original Reno.
13  */
14 
15 #include <linux/config.h>
16 #include <linux/mm.h>
17 #include <linux/module.h>
18 #include <net/tcp.h>
19 
20 
21 #define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
22 					 * max_cwnd = snd_cwnd * beta
23 					 */
24 #define BICTCP_B		4	 /*
25 					  * In binary search,
26 					  * go to point (max+min)/N
27 					  */
28 
29 static int fast_convergence = 1;
30 static int max_increment = 16;
31 static int low_window = 14;
32 static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
33 static int low_utilization_threshold = 153;
34 static int low_utilization_period = 2;
35 static int initial_ssthresh = 100;
36 static int smooth_part = 20;
37 
38 module_param(fast_convergence, int, 0644);
39 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
40 module_param(max_increment, int, 0644);
41 MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
42 module_param(low_window, int, 0644);
43 MODULE_PARM_DESC(low_window, "lower bound on congestion window (for TCP friendliness)");
44 module_param(beta, int, 0644);
45 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
46 module_param(low_utilization_threshold, int, 0644);
47 MODULE_PARM_DESC(low_utilization_threshold, "percent (scaled by 1024) for low utilization mode");
48 module_param(low_utilization_period, int, 0644);
49 MODULE_PARM_DESC(low_utilization_period, "if average delay exceeds then goto to low utilization mode (seconds)");
50 module_param(initial_ssthresh, int, 0644);
51 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
52 module_param(smooth_part, int, 0644);
53 MODULE_PARM_DESC(smooth_part, "log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax");
54 
55 
56 /* BIC TCP Parameters */
57 struct bictcp {
58 	u32	cnt;		/* increase cwnd by 1 after ACKs */
59 	u32 	last_max_cwnd;	/* last maximum snd_cwnd */
60 	u32	loss_cwnd;	/* congestion window at last loss */
61 	u32	last_cwnd;	/* the last snd_cwnd */
62 	u32	last_time;	/* time when updated last_cwnd */
63 	u32	delay_min;	/* min delay */
64 	u32	delay_max;	/* max delay */
65 	u32	last_delay;
66 	u8	low_utilization;/* 0: high; 1: low */
67 	u32	low_utilization_start;	/* starting time of low utilization detection*/
68 	u32	epoch_start;	/* beginning of an epoch */
69 #define ACK_RATIO_SHIFT	4
70 	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
71 };
72 
73 static inline void bictcp_reset(struct bictcp *ca)
74 {
75 	ca->cnt = 0;
76 	ca->last_max_cwnd = 0;
77 	ca->loss_cwnd = 0;
78 	ca->last_cwnd = 0;
79 	ca->last_time = 0;
80 	ca->delay_min = 0;
81 	ca->delay_max = 0;
82 	ca->last_delay = 0;
83 	ca->low_utilization = 0;
84 	ca->low_utilization_start = 0;
85 	ca->epoch_start = 0;
86 	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
87 }
88 
89 static void bictcp_init(struct sock *sk)
90 {
91 	bictcp_reset(inet_csk_ca(sk));
92 	if (initial_ssthresh)
93 		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
94 }
95 
96 /*
97  * Compute congestion window to use.
98  */
99 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
100 {
101 	if (ca->last_cwnd == cwnd &&
102 	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
103 		return;
104 
105 	ca->last_cwnd = cwnd;
106 	ca->last_time = tcp_time_stamp;
107 
108 	if (ca->epoch_start == 0) /* record the beginning of an epoch */
109 		ca->epoch_start = tcp_time_stamp;
110 
111 	/* start off normal */
112 	if (cwnd <= low_window) {
113 		ca->cnt = cwnd;
114 		return;
115 	}
116 
117 	/* binary increase */
118 	if (cwnd < ca->last_max_cwnd) {
119 		__u32 	dist = (ca->last_max_cwnd - cwnd)
120 			/ BICTCP_B;
121 
122 		if (dist > max_increment)
123 			/* linear increase */
124 			ca->cnt = cwnd / max_increment;
125 		else if (dist <= 1U)
126 			/* binary search increase */
127 			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
128 		else
129 			/* binary search increase */
130 			ca->cnt = cwnd / dist;
131 	} else {
132 		/* slow start AMD linear increase */
133 		if (cwnd < ca->last_max_cwnd + BICTCP_B)
134 			/* slow start */
135 			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
136 		else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
137 			/* slow start */
138 			ca->cnt = (cwnd * (BICTCP_B-1))
139 				/ (cwnd - ca->last_max_cwnd);
140 		else
141 			/* linear increase */
142 			ca->cnt = cwnd / max_increment;
143 	}
144 
145 	/* if in slow start or link utilization is very low */
146 	if ( ca->loss_cwnd == 0 ||
147 	     (cwnd > ca->loss_cwnd && ca->low_utilization)) {
148 		if (ca->cnt > 20) /* increase cwnd 5% per RTT */
149 			ca->cnt = 20;
150 	}
151 
152 	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
153 	if (ca->cnt == 0)			/* cannot be zero */
154 		ca->cnt = 1;
155 }
156 
157 
158 /* Detect low utilization in congestion avoidance */
159 static inline void bictcp_low_utilization(struct sock *sk, int flag)
160 {
161 	const struct tcp_sock *tp = tcp_sk(sk);
162 	struct bictcp *ca = inet_csk_ca(sk);
163 	u32 dist, delay;
164 
165 	/* No time stamp */
166 	if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) ||
167 	     /* Discard delay samples right after fast recovery */
168 	     tcp_time_stamp < ca->epoch_start + HZ ||
169 	     /* this delay samples may not be accurate */
170 	     flag == 0) {
171 		ca->last_delay = 0;
172 		goto notlow;
173 	}
174 
175 	delay = ca->last_delay<<3;	/* use the same scale as tp->srtt*/
176 	ca->last_delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
177 	if (delay == 0) 		/* no previous delay sample */
178 		goto notlow;
179 
180 	/* first time call or link delay decreases */
181 	if (ca->delay_min == 0 || ca->delay_min > delay) {
182 		ca->delay_min = ca->delay_max = delay;
183 		goto notlow;
184 	}
185 
186 	if (ca->delay_max < delay)
187 		ca->delay_max = delay;
188 
189 	/* utilization is low, if avg delay < dist*threshold
190 	   for checking_period time */
191 	dist = ca->delay_max - ca->delay_min;
192 	if (dist <= ca->delay_min>>6 ||
193 	    tp->srtt - ca->delay_min >=  (dist*low_utilization_threshold)>>10)
194 		goto notlow;
195 
196 	if (ca->low_utilization_start == 0) {
197 		ca->low_utilization = 0;
198 		ca->low_utilization_start = tcp_time_stamp;
199 	} else if ((s32)(tcp_time_stamp - ca->low_utilization_start)
200 			> low_utilization_period*HZ) {
201 		ca->low_utilization = 1;
202 	}
203 
204 	return;
205 
206  notlow:
207 	ca->low_utilization = 0;
208 	ca->low_utilization_start = 0;
209 
210 }
211 
212 static void bictcp_cong_avoid(struct sock *sk, u32 ack,
213 			      u32 seq_rtt, u32 in_flight, int data_acked)
214 {
215 	struct tcp_sock *tp = tcp_sk(sk);
216 	struct bictcp *ca = inet_csk_ca(sk);
217 
218 	bictcp_low_utilization(sk, data_acked);
219 
220 	if (in_flight < tp->snd_cwnd)
221 		return;
222 
223 	if (tp->snd_cwnd <= tp->snd_ssthresh) {
224 		/* In "safe" area, increase. */
225 		if (tp->snd_cwnd < tp->snd_cwnd_clamp)
226 			tp->snd_cwnd++;
227 	} else {
228 		bictcp_update(ca, tp->snd_cwnd);
229 
230                 /* In dangerous area, increase slowly.
231 		 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
232 		 */
233 		if (tp->snd_cwnd_cnt >= ca->cnt) {
234 			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
235 				tp->snd_cwnd++;
236 			tp->snd_cwnd_cnt = 0;
237 		} else
238 			tp->snd_cwnd_cnt++;
239 	}
240 
241 }
242 
243 /*
244  *	behave like Reno until low_window is reached,
245  *	then increase congestion window slowly
246  */
247 static u32 bictcp_recalc_ssthresh(struct sock *sk)
248 {
249 	const struct tcp_sock *tp = tcp_sk(sk);
250 	struct bictcp *ca = inet_csk_ca(sk);
251 
252 	ca->epoch_start = 0;	/* end of epoch */
253 
254 	/* in case of wrong delay_max*/
255 	if (ca->delay_min > 0 && ca->delay_max > ca->delay_min)
256 		ca->delay_max = ca->delay_min
257 			+ ((ca->delay_max - ca->delay_min)* 90) / 100;
258 
259 	/* Wmax and fast convergence */
260 	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
261 		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
262 			/ (2 * BICTCP_BETA_SCALE);
263 	else
264 		ca->last_max_cwnd = tp->snd_cwnd;
265 
266 	ca->loss_cwnd = tp->snd_cwnd;
267 
268 
269 	if (tp->snd_cwnd <= low_window)
270 		return max(tp->snd_cwnd >> 1U, 2U);
271 	else
272 		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
273 }
274 
275 static u32 bictcp_undo_cwnd(struct sock *sk)
276 {
277 	const struct tcp_sock *tp = tcp_sk(sk);
278 	const struct bictcp *ca = inet_csk_ca(sk);
279 	return max(tp->snd_cwnd, ca->last_max_cwnd);
280 }
281 
282 static u32 bictcp_min_cwnd(struct sock *sk)
283 {
284 	const struct tcp_sock *tp = tcp_sk(sk);
285 	return tp->snd_ssthresh;
286 }
287 
288 static void bictcp_state(struct sock *sk, u8 new_state)
289 {
290 	if (new_state == TCP_CA_Loss)
291 		bictcp_reset(inet_csk_ca(sk));
292 }
293 
294 /* Track delayed acknowledgement ratio using sliding window
295  * ratio = (15*ratio + sample) / 16
296  */
297 static void bictcp_acked(struct sock *sk, u32 cnt)
298 {
299 	const struct inet_connection_sock *icsk = inet_csk(sk);
300 
301 	if (cnt > 0 && 	icsk->icsk_ca_state == TCP_CA_Open) {
302 		struct bictcp *ca = inet_csk_ca(sk);
303 		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
304 		ca->delayed_ack += cnt;
305 	}
306 }
307 
308 
309 static struct tcp_congestion_ops bictcp = {
310 	.init		= bictcp_init,
311 	.ssthresh	= bictcp_recalc_ssthresh,
312 	.cong_avoid	= bictcp_cong_avoid,
313 	.set_state	= bictcp_state,
314 	.undo_cwnd	= bictcp_undo_cwnd,
315 	.min_cwnd	= bictcp_min_cwnd,
316 	.pkts_acked     = bictcp_acked,
317 	.owner		= THIS_MODULE,
318 	.name		= "bic",
319 };
320 
321 static int __init bictcp_register(void)
322 {
323 	BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
324 	return tcp_register_congestion_control(&bictcp);
325 }
326 
327 static void __exit bictcp_unregister(void)
328 {
329 	tcp_unregister_congestion_control(&bictcp);
330 }
331 
332 module_init(bictcp_register);
333 module_exit(bictcp_unregister);
334 
335 MODULE_AUTHOR("Stephen Hemminger");
336 MODULE_LICENSE("GPL");
337 MODULE_DESCRIPTION("BIC TCP");
338