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 */
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. */
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
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 */
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
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 */
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
160 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
161 {
162 return min(tp->snd_ssthresh, tp->snd_cwnd);
163 }
164
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)tp->snd_cwnd * 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 = tp->snd_cwnd * (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 tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);
242 tp->snd_ssthresh = tcp_vegas_ssthresh(tp);
243
244 } else if (tcp_in_slow_start(tp)) {
245 /* Slow start. */
246 tcp_slow_start(tp, acked);
247 } else {
248 /* Congestion avoidance. */
249
250 /* Figure out where we would like cwnd
251 * to be.
252 */
253 if (diff > beta) {
254 /* The old window was too fast, so
255 * we slow down.
256 */
257 tp->snd_cwnd--;
258 tp->snd_ssthresh
259 = tcp_vegas_ssthresh(tp);
260 } else if (diff < alpha) {
261 /* We don't have enough extra packets
262 * in the network, so speed up.
263 */
264 tp->snd_cwnd++;
265 } else {
266 /* Sending just as fast as we
267 * should be.
268 */
269 }
270 }
271
272 if (tp->snd_cwnd < 2)
273 tp->snd_cwnd = 2;
274 else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
275 tp->snd_cwnd = tp->snd_cwnd_clamp;
276
277 tp->snd_ssthresh = tcp_current_ssthresh(sk);
278 }
279
280 /* Wipe the slate clean for the next RTT. */
281 vegas->cntRTT = 0;
282 vegas->minRTT = 0x7fffffff;
283 }
284 /* Use normal slow start */
285 else if (tcp_in_slow_start(tp))
286 tcp_slow_start(tp, acked);
287 }
288
289 /* Extract info for Tcp socket info provided via netlink. */
290 size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
291 union tcp_cc_info *info)
292 {
293 const struct vegas *ca = inet_csk_ca(sk);
294
295 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
296 info->vegas.tcpv_enabled = ca->doing_vegas_now,
297 info->vegas.tcpv_rttcnt = ca->cntRTT,
298 info->vegas.tcpv_rtt = ca->baseRTT,
299 info->vegas.tcpv_minrtt = ca->minRTT,
300
301 *attr = INET_DIAG_VEGASINFO;
302 return sizeof(struct tcpvegas_info);
303 }
304 return 0;
305 }
306 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
307
308 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
309 .init = tcp_vegas_init,
310 .ssthresh = tcp_reno_ssthresh,
311 .undo_cwnd = tcp_reno_undo_cwnd,
312 .cong_avoid = tcp_vegas_cong_avoid,
313 .pkts_acked = tcp_vegas_pkts_acked,
314 .set_state = tcp_vegas_state,
315 .cwnd_event = tcp_vegas_cwnd_event,
316 .get_info = tcp_vegas_get_info,
317
318 .owner = THIS_MODULE,
319 .name = "vegas",
320 };
321
322 static int __init tcp_vegas_register(void)
323 {
324 BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
325 tcp_register_congestion_control(&tcp_vegas);
326 return 0;
327 }
328
329 static void __exit tcp_vegas_unregister(void)
330 {
331 tcp_unregister_congestion_control(&tcp_vegas);
332 }
333
334 module_init(tcp_vegas_register);
335 module_exit(tcp_vegas_unregister);
336
337 MODULE_AUTHOR("Stephen Hemminger");
338 MODULE_LICENSE("GPL");
339 MODULE_DESCRIPTION("TCP Vegas");