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