1 /*
2 * TCP Vegas congestion control
3 *
4 * This is based on the congestion detection/avoidance scheme described in
5 * Lawrence S. Brakmo and Larry L. Peterson.
6 * "TCP Vegas: End to end congestion avoidance on a global internet."
7 * IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
8 * October 1995. Available from:
9 * ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
10 *
11 * See http://www.cs.arizona.edu/xkernel/ for their implementation.
12 * The main aspects that distinguish this implementation from the
13 * Arizona Vegas implementation are:
14 * o We do not change the loss detection or recovery mechanisms of
15 * Linux in any way. Linux already recovers from losses quite well,
16 * using fine-grained timers, NewReno, and FACK.
17 * o To avoid the performance penalty imposed by increasing cwnd
18 * only every-other RTT during slow start, we increase during
19 * every RTT during slow start, just like Reno.
20 * o Largely to allow continuous cwnd growth during slow start,
21 * we use the rate at which ACKs come back as the "actual"
22 * rate, rather than the rate at which data is sent.
23 * o To speed convergence to the right rate, we set the cwnd
24 * to achieve the right ("actual") rate when we exit slow start.
25 * o To filter out the noise caused by delayed ACKs, we use the
26 * minimum RTT sample observed during the last RTT to calculate
27 * the actual rate.
28 * o When the sender re-starts from idle, it waits until it has
29 * received ACKs for an entire flight of new data before making
30 * a cwnd adjustment decision. The original Vegas implementation
31 * assumed senders never went idle.
32 */
33
34 #include <linux/mm.h>
35 #include <linux/module.h>
36 #include <linux/skbuff.h>
37 #include <linux/inet_diag.h>
38
39 #include <net/tcp.h>
40
41 #include "tcp_vegas.h"
42
43 static int alpha = 2;
44 static int beta = 4;
45 static int gamma = 1;
46
47 module_param(alpha, int, 0644);
48 MODULE_PARM_DESC(alpha, "lower bound of packets in network");
49 module_param(beta, int, 0644);
50 MODULE_PARM_DESC(beta, "upper bound of packets in network");
51 module_param(gamma, int, 0644);
52 MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
53
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,u32 cnt,s32 rtt_us)111 void tcp_vegas_pkts_acked(struct sock *sk, u32 cnt, s32 rtt_us)
112 {
113 struct vegas *vegas = inet_csk_ca(sk);
114 u32 vrtt;
115
116 if (rtt_us < 0)
117 return;
118
119 /* Never allow zero rtt or baseRTT */
120 vrtt = 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
137 if (ca_state == TCP_CA_Open)
138 vegas_enable(sk);
139 else
140 vegas_disable(sk);
141 }
142 EXPORT_SYMBOL_GPL(tcp_vegas_state);
143
144 /*
145 * If the connection is idle and we are restarting,
146 * then we don't want to do any Vegas calculations
147 * until we get fresh RTT samples. So when we
148 * restart, we reset our Vegas state to a clean
149 * slate. After we get acks for this flight of
150 * packets, _then_ we can make Vegas calculations
151 * again.
152 */
tcp_vegas_cwnd_event(struct sock * sk,enum tcp_ca_event event)153 void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
154 {
155 if (event == CA_EVENT_CWND_RESTART ||
156 event == CA_EVENT_TX_START)
157 tcp_vegas_init(sk);
158 }
159 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
160
tcp_vegas_cong_avoid(struct sock * sk,u32 ack,u32 in_flight)161 static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
162 {
163 struct tcp_sock *tp = tcp_sk(sk);
164 struct vegas *vegas = inet_csk_ca(sk);
165
166 if (!vegas->doing_vegas_now) {
167 tcp_reno_cong_avoid(sk, ack, in_flight);
168 return;
169 }
170
171 if (after(ack, vegas->beg_snd_nxt)) {
172 /* Do the Vegas once-per-RTT cwnd adjustment. */
173
174 /* Save the extent of the current window so we can use this
175 * at the end of the next RTT.
176 */
177 vegas->beg_snd_nxt = tp->snd_nxt;
178
179 /* We do the Vegas calculations only if we got enough RTT
180 * samples that we can be reasonably sure that we got
181 * at least one RTT sample that wasn't from a delayed ACK.
182 * If we only had 2 samples total,
183 * then that means we're getting only 1 ACK per RTT, which
184 * means they're almost certainly delayed ACKs.
185 * If we have 3 samples, we should be OK.
186 */
187
188 if (vegas->cntRTT <= 2) {
189 /* We don't have enough RTT samples to do the Vegas
190 * calculation, so we'll behave like Reno.
191 */
192 tcp_reno_cong_avoid(sk, ack, in_flight);
193 } else {
194 u32 rtt, diff;
195 u64 target_cwnd;
196
197 /* We have enough RTT samples, so, using the Vegas
198 * algorithm, we determine if we should increase or
199 * decrease cwnd, and by how much.
200 */
201
202 /* Pluck out the RTT we are using for the Vegas
203 * calculations. This is the min RTT seen during the
204 * last RTT. Taking the min filters out the effects
205 * of delayed ACKs, at the cost of noticing congestion
206 * a bit later.
207 */
208 rtt = vegas->minRTT;
209
210 /* Calculate the cwnd we should have, if we weren't
211 * going too fast.
212 *
213 * This is:
214 * (actual rate in segments) * baseRTT
215 */
216 target_cwnd = tp->snd_cwnd * vegas->baseRTT / rtt;
217
218 /* Calculate the difference between the window we had,
219 * and the window we would like to have. This quantity
220 * is the "Diff" from the Arizona Vegas papers.
221 */
222 diff = tp->snd_cwnd * (rtt-vegas->baseRTT) / vegas->baseRTT;
223
224 if (diff > gamma && tp->snd_ssthresh > 2 ) {
225 /* Going too fast. Time to slow down
226 * and switch to congestion avoidance.
227 */
228 tp->snd_ssthresh = 2;
229
230 /* Set cwnd to match the actual rate
231 * exactly:
232 * cwnd = (actual rate) * baseRTT
233 * Then we add 1 because the integer
234 * truncation robs us of full link
235 * utilization.
236 */
237 tp->snd_cwnd = min(tp->snd_cwnd, (u32)target_cwnd+1);
238
239 } else if (tp->snd_cwnd <= tp->snd_ssthresh) {
240 /* Slow start. */
241 tcp_slow_start(tp);
242 } else {
243 /* Congestion avoidance. */
244
245 /* Figure out where we would like cwnd
246 * to be.
247 */
248 if (diff > beta) {
249 /* The old window was too fast, so
250 * we slow down.
251 */
252 tp->snd_cwnd--;
253 } else if (diff < alpha) {
254 /* We don't have enough extra packets
255 * in the network, so speed up.
256 */
257 tp->snd_cwnd++;
258 } else {
259 /* Sending just as fast as we
260 * should be.
261 */
262 }
263 }
264
265 if (tp->snd_cwnd < 2)
266 tp->snd_cwnd = 2;
267 else if (tp->snd_cwnd > tp->snd_cwnd_clamp)
268 tp->snd_cwnd = tp->snd_cwnd_clamp;
269
270 tp->snd_ssthresh = tcp_current_ssthresh(sk);
271 }
272
273 /* Wipe the slate clean for the next RTT. */
274 vegas->cntRTT = 0;
275 vegas->minRTT = 0x7fffffff;
276 }
277 /* Use normal slow start */
278 else if (tp->snd_cwnd <= tp->snd_ssthresh)
279 tcp_slow_start(tp);
280
281 }
282
283 /* Extract info for Tcp socket info provided via netlink. */
tcp_vegas_get_info(struct sock * sk,u32 ext,struct sk_buff * skb)284 void tcp_vegas_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
285 {
286 const struct vegas *ca = inet_csk_ca(sk);
287 if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
288 struct tcpvegas_info info = {
289 .tcpv_enabled = ca->doing_vegas_now,
290 .tcpv_rttcnt = ca->cntRTT,
291 .tcpv_rtt = ca->baseRTT,
292 .tcpv_minrtt = ca->minRTT,
293 };
294
295 nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
296 }
297 }
298 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
299
300 static struct tcp_congestion_ops tcp_vegas = {
301 .flags = TCP_CONG_RTT_STAMP,
302 .init = tcp_vegas_init,
303 .ssthresh = tcp_reno_ssthresh,
304 .cong_avoid = tcp_vegas_cong_avoid,
305 .min_cwnd = tcp_reno_min_cwnd,
306 .pkts_acked = tcp_vegas_pkts_acked,
307 .set_state = tcp_vegas_state,
308 .cwnd_event = tcp_vegas_cwnd_event,
309 .get_info = tcp_vegas_get_info,
310
311 .owner = THIS_MODULE,
312 .name = "vegas",
313 };
314
tcp_vegas_register(void)315 static int __init tcp_vegas_register(void)
316 {
317 BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
318 tcp_register_congestion_control(&tcp_vegas);
319 return 0;
320 }
321
tcp_vegas_unregister(void)322 static void __exit tcp_vegas_unregister(void)
323 {
324 tcp_unregister_congestion_control(&tcp_vegas);
325 }
326
327 module_init(tcp_vegas_register);
328 module_exit(tcp_vegas_unregister);
329
330 MODULE_AUTHOR("Stephen Hemminger");
331 MODULE_LICENSE("GPL");
332 MODULE_DESCRIPTION("TCP Vegas");
333