xref: /openbmc/linux/net/ipv4/tcp_vegas.c (revision 3308676e)
1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3  * TCP Vegas congestion control
4  *
5  * This is based on the congestion detection/avoidance scheme described in
6  *    Lawrence S. Brakmo and Larry L. Peterson.
7  *    "TCP Vegas: End to end congestion avoidance on a global internet."
8  *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
9  *    October 1995. Available from:
10  *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
11  *
12  * See http://www.cs.arizona.edu/xkernel/ for their implementation.
13  * The main aspects that distinguish this implementation from the
14  * Arizona Vegas implementation are:
15  *   o We do not change the loss detection or recovery mechanisms of
16  *     Linux in any way. Linux already recovers from losses quite well,
17  *     using fine-grained timers, NewReno, and FACK.
18  *   o To avoid the performance penalty imposed by increasing cwnd
19  *     only every-other RTT during slow start, we increase during
20  *     every RTT during slow start, just like Reno.
21  *   o Largely to allow continuous cwnd growth during slow start,
22  *     we use the rate at which ACKs come back as the "actual"
23  *     rate, rather than the rate at which data is sent.
24  *   o To speed convergence to the right rate, we set the cwnd
25  *     to achieve the right ("actual") rate when we exit slow start.
26  *   o To filter out the noise caused by delayed ACKs, we use the
27  *     minimum RTT sample observed during the last RTT to calculate
28  *     the actual rate.
29  *   o When the sender re-starts from idle, it waits until it has
30  *     received ACKs for an entire flight of new data before making
31  *     a cwnd adjustment decision. The original Vegas implementation
32  *     assumed senders never went idle.
33  */
34 
35 #include <linux/mm.h>
36 #include <linux/module.h>
37 #include <linux/skbuff.h>
38 #include <linux/inet_diag.h>
39 
40 #include <net/tcp.h>
41 
42 #include "tcp_vegas.h"
43 
44 static int alpha = 2;
45 static int beta  = 4;
46 static int gamma = 1;
47 
48 module_param(alpha, int, 0644);
49 MODULE_PARM_DESC(alpha, "lower bound of packets in network");
50 module_param(beta, int, 0644);
51 MODULE_PARM_DESC(beta, "upper bound of packets in network");
52 module_param(gamma, int, 0644);
53 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
54 
55 /* There are several situations when we must "re-start" Vegas:
56  *
57  *  o when a connection is established
58  *  o after an RTO
59  *  o after fast recovery
60  *  o when we send a packet and there is no outstanding
61  *    unacknowledged data (restarting an idle connection)
62  *
63  * In these circumstances we cannot do a Vegas calculation at the
64  * end of the first RTT, because any calculation we do is using
65  * stale info -- both the saved cwnd and congestion feedback are
66  * stale.
67  *
68  * Instead we must wait until the completion of an RTT during
69  * which we actually receive ACKs.
70  */
vegas_enable(struct sock * sk)71 static void vegas_enable(struct sock *sk)
72 {
73 	const struct tcp_sock *tp = tcp_sk(sk);
74 	struct vegas *vegas = inet_csk_ca(sk);
75 
76 	/* Begin taking Vegas samples next time we send something. */
77 	vegas->doing_vegas_now = 1;
78 
79 	/* Set the beginning of the next send window. */
80 	vegas->beg_snd_nxt = tp->snd_nxt;
81 
82 	vegas->cntRTT = 0;
83 	vegas->minRTT = 0x7fffffff;
84 }
85 
86 /* Stop taking Vegas samples for now. */
vegas_disable(struct sock * sk)87 static inline void vegas_disable(struct sock *sk)
88 {
89 	struct vegas *vegas = inet_csk_ca(sk);
90 
91 	vegas->doing_vegas_now = 0;
92 }
93 
tcp_vegas_init(struct sock * sk)94 void tcp_vegas_init(struct sock *sk)
95 {
96 	struct vegas *vegas = inet_csk_ca(sk);
97 
98 	vegas->baseRTT = 0x7fffffff;
99 	vegas_enable(sk);
100 }
101 EXPORT_SYMBOL_GPL(tcp_vegas_init);
102 
103 /* Do RTT sampling needed for Vegas.
104  * Basically we:
105  *   o min-filter RTT samples from within an RTT to get the current
106  *     propagation delay + queuing delay (we are min-filtering to try to
107  *     avoid the effects of delayed ACKs)
108  *   o min-filter RTT samples from a much longer window (forever for now)
109  *     to find the propagation delay (baseRTT)
110  */
tcp_vegas_pkts_acked(struct sock * sk,const struct ack_sample * sample)111 void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample)
112 {
113 	struct vegas *vegas = inet_csk_ca(sk);
114 	u32 vrtt;
115 
116 	if (sample->rtt_us < 0)
117 		return;
118 
119 	/* Never allow zero rtt or baseRTT */
120 	vrtt = sample->rtt_us + 1;
121 
122 	/* Filter to find propagation delay: */
123 	if (vrtt < vegas->baseRTT)
124 		vegas->baseRTT = vrtt;
125 
126 	/* Find the min RTT during the last RTT to find
127 	 * the current prop. delay + queuing delay:
128 	 */
129 	vegas->minRTT = min(vegas->minRTT, vrtt);
130 	vegas->cntRTT++;
131 }
132 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);
133 
tcp_vegas_state(struct sock * sk,u8 ca_state)134 void tcp_vegas_state(struct sock *sk, u8 ca_state)
135 {
136 	if (ca_state == TCP_CA_Open)
137 		vegas_enable(sk);
138 	else
139 		vegas_disable(sk);
140 }
141 EXPORT_SYMBOL_GPL(tcp_vegas_state);
142 
143 /*
144  * If the connection is idle and we are restarting,
145  * then we don't want to do any Vegas calculations
146  * until we get fresh RTT samples.  So when we
147  * restart, we reset our Vegas state to a clean
148  * slate. After we get acks for this flight of
149  * packets, _then_ we can make Vegas calculations
150  * again.
151  */
tcp_vegas_cwnd_event(struct sock * sk,enum tcp_ca_event event)152 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
153 {
154 	if (event == CA_EVENT_CWND_RESTART ||
155 	    event == CA_EVENT_TX_START)
156 		tcp_vegas_init(sk);
157 }
158 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
159 
tcp_vegas_ssthresh(struct tcp_sock * tp)160 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
161 {
162 	return  min(tp->snd_ssthresh, tcp_snd_cwnd(tp));
163 }
164 
tcp_vegas_cong_avoid(struct sock * sk,u32 ack,u32 acked)165 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
166 {
167 	struct tcp_sock *tp = tcp_sk(sk);
168 	struct vegas *vegas = inet_csk_ca(sk);
169 
170 	if (!vegas->doing_vegas_now) {
171 		tcp_reno_cong_avoid(sk, ack, acked);
172 		return;
173 	}
174 
175 	if (after(ack, vegas->beg_snd_nxt)) {
176 		/* Do the Vegas once-per-RTT cwnd adjustment. */
177 
178 		/* Save the extent of the current window so we can use this
179 		 * at the end of the next RTT.
180 		 */
181 		vegas->beg_snd_nxt  = tp->snd_nxt;
182 
183 		/* We do the Vegas calculations only if we got enough RTT
184 		 * samples that we can be reasonably sure that we got
185 		 * at least one RTT sample that wasn't from a delayed ACK.
186 		 * If we only had 2 samples total,
187 		 * then that means we're getting only 1 ACK per RTT, which
188 		 * means they're almost certainly delayed ACKs.
189 		 * If  we have 3 samples, we should be OK.
190 		 */
191 
192 		if (vegas->cntRTT <= 2) {
193 			/* We don't have enough RTT samples to do the Vegas
194 			 * calculation, so we'll behave like Reno.
195 			 */
196 			tcp_reno_cong_avoid(sk, ack, acked);
197 		} else {
198 			u32 rtt, diff;
199 			u64 target_cwnd;
200 
201 			/* We have enough RTT samples, so, using the Vegas
202 			 * algorithm, we determine if we should increase or
203 			 * decrease cwnd, and by how much.
204 			 */
205 
206 			/* Pluck out the RTT we are using for the Vegas
207 			 * calculations. This is the min RTT seen during the
208 			 * last RTT. Taking the min filters out the effects
209 			 * of delayed ACKs, at the cost of noticing congestion
210 			 * a bit later.
211 			 */
212 			rtt = vegas->minRTT;
213 
214 			/* Calculate the cwnd we should have, if we weren't
215 			 * going too fast.
216 			 *
217 			 * This is:
218 			 *     (actual rate in segments) * baseRTT
219 			 */
220 			target_cwnd = (u64)tcp_snd_cwnd(tp) * vegas->baseRTT;
221 			do_div(target_cwnd, rtt);
222 
223 			/* Calculate the difference between the window we had,
224 			 * and the window we would like to have. This quantity
225 			 * is the "Diff" from the Arizona Vegas papers.
226 			 */
227 			diff = tcp_snd_cwnd(tp) * (rtt-vegas->baseRTT) / vegas->baseRTT;
228 
229 			if (diff > gamma && tcp_in_slow_start(tp)) {
230 				/* Going too fast. Time to slow down
231 				 * and switch to congestion avoidance.
232 				 */
233 
234 				/* Set cwnd to match the actual rate
235 				 * exactly:
236 				 *   cwnd = (actual rate) * baseRTT
237 				 * Then we add 1 because the integer
238 				 * truncation robs us of full link
239 				 * utilization.
240 				 */
241 				tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp),
242 							 (u32)target_cwnd + 1));
243 				tp->snd_ssthresh = tcp_vegas_ssthresh(tp);
244 
245 			} else if (tcp_in_slow_start(tp)) {
246 				/* Slow start.  */
247 				tcp_slow_start(tp, acked);
248 			} else {
249 				/* Congestion avoidance. */
250 
251 				/* Figure out where we would like cwnd
252 				 * to be.
253 				 */
254 				if (diff > beta) {
255 					/* The old window was too fast, so
256 					 * we slow down.
257 					 */
258 					tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
259 					tp->snd_ssthresh
260 						= tcp_vegas_ssthresh(tp);
261 				} else if (diff < alpha) {
262 					/* We don't have enough extra packets
263 					 * in the network, so speed up.
264 					 */
265 					tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
266 				} else {
267 					/* Sending just as fast as we
268 					 * should be.
269 					 */
270 				}
271 			}
272 
273 			if (tcp_snd_cwnd(tp) < 2)
274 				tcp_snd_cwnd_set(tp, 2);
275 			else if (tcp_snd_cwnd(tp) > tp->snd_cwnd_clamp)
276 				tcp_snd_cwnd_set(tp, tp->snd_cwnd_clamp);
277 
278 			tp->snd_ssthresh = tcp_current_ssthresh(sk);
279 		}
280 
281 		/* Wipe the slate clean for the next RTT. */
282 		vegas->cntRTT = 0;
283 		vegas->minRTT = 0x7fffffff;
284 	}
285 	/* Use normal slow start */
286 	else if (tcp_in_slow_start(tp))
287 		tcp_slow_start(tp, acked);
288 }
289 
290 /* Extract info for Tcp socket info provided via netlink. */
tcp_vegas_get_info(struct sock * sk,u32 ext,int * attr,union tcp_cc_info * info)291 size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
292 			  union tcp_cc_info *info)
293 {
294 	const struct vegas *ca = inet_csk_ca(sk);
295 
296 	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
297 		info->vegas.tcpv_enabled = ca->doing_vegas_now;
298 		info->vegas.tcpv_rttcnt = ca->cntRTT;
299 		info->vegas.tcpv_rtt = ca->baseRTT;
300 		info->vegas.tcpv_minrtt = ca->minRTT;
301 
302 		*attr = INET_DIAG_VEGASINFO;
303 		return sizeof(struct tcpvegas_info);
304 	}
305 	return 0;
306 }
307 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
308 
309 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
310 	.init		= tcp_vegas_init,
311 	.ssthresh	= tcp_reno_ssthresh,
312 	.undo_cwnd	= tcp_reno_undo_cwnd,
313 	.cong_avoid	= tcp_vegas_cong_avoid,
314 	.pkts_acked	= tcp_vegas_pkts_acked,
315 	.set_state	= tcp_vegas_state,
316 	.cwnd_event	= tcp_vegas_cwnd_event,
317 	.get_info	= tcp_vegas_get_info,
318 
319 	.owner		= THIS_MODULE,
320 	.name		= "vegas",
321 };
322 
tcp_vegas_register(void)323 static int __init tcp_vegas_register(void)
324 {
325 	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
326 	tcp_register_congestion_control(&tcp_vegas);
327 	return 0;
328 }
329 
tcp_vegas_unregister(void)330 static void __exit tcp_vegas_unregister(void)
331 {
332 	tcp_unregister_congestion_control(&tcp_vegas);
333 }
334 
335 module_init(tcp_vegas_register);
336 module_exit(tcp_vegas_unregister);
337 
338 MODULE_AUTHOR("Stephen Hemminger");
339 MODULE_LICENSE("GPL");
340 MODULE_DESCRIPTION("TCP Vegas");
341