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b87d8561 SH |
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/config.h> | |
35 | #include <linux/mm.h> | |
36 | #include <linux/module.h> | |
37 | #include <linux/skbuff.h> | |
38 | #include <linux/tcp_diag.h> | |
39 | ||
40 | #include <net/tcp.h> | |
41 | ||
42 | /* Default values of the Vegas variables, in fixed-point representation | |
43 | * with V_PARAM_SHIFT bits to the right of the binary point. | |
44 | */ | |
45 | #define V_PARAM_SHIFT 1 | |
46 | static int alpha = 1<<V_PARAM_SHIFT; | |
47 | static int beta = 3<<V_PARAM_SHIFT; | |
48 | static int gamma = 1<<V_PARAM_SHIFT; | |
49 | ||
50 | module_param(alpha, int, 0644); | |
51 | MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)"); | |
52 | module_param(beta, int, 0644); | |
53 | MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)"); | |
54 | module_param(gamma, int, 0644); | |
55 | MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)"); | |
56 | ||
57 | ||
58 | /* Vegas variables */ | |
59 | struct vegas { | |
60 | u32 beg_snd_nxt; /* right edge during last RTT */ | |
61 | u32 beg_snd_una; /* left edge during last RTT */ | |
62 | u32 beg_snd_cwnd; /* saves the size of the cwnd */ | |
63 | u8 doing_vegas_now;/* if true, do vegas for this RTT */ | |
64 | u16 cntRTT; /* # of RTTs measured within last RTT */ | |
65 | u32 minRTT; /* min of RTTs measured within last RTT (in usec) */ | |
66 | u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */ | |
67 | }; | |
68 | ||
69 | /* There are several situations when we must "re-start" Vegas: | |
70 | * | |
71 | * o when a connection is established | |
72 | * o after an RTO | |
73 | * o after fast recovery | |
74 | * o when we send a packet and there is no outstanding | |
75 | * unacknowledged data (restarting an idle connection) | |
76 | * | |
77 | * In these circumstances we cannot do a Vegas calculation at the | |
78 | * end of the first RTT, because any calculation we do is using | |
79 | * stale info -- both the saved cwnd and congestion feedback are | |
80 | * stale. | |
81 | * | |
82 | * Instead we must wait until the completion of an RTT during | |
83 | * which we actually receive ACKs. | |
84 | */ | |
6687e988 | 85 | static inline void vegas_enable(struct sock *sk) |
b87d8561 | 86 | { |
6687e988 ACM |
87 | const struct tcp_sock *tp = tcp_sk(sk); |
88 | struct vegas *vegas = inet_csk_ca(sk); | |
b87d8561 SH |
89 | |
90 | /* Begin taking Vegas samples next time we send something. */ | |
91 | vegas->doing_vegas_now = 1; | |
92 | ||
93 | /* Set the beginning of the next send window. */ | |
94 | vegas->beg_snd_nxt = tp->snd_nxt; | |
95 | ||
96 | vegas->cntRTT = 0; | |
97 | vegas->minRTT = 0x7fffffff; | |
98 | } | |
99 | ||
100 | /* Stop taking Vegas samples for now. */ | |
6687e988 | 101 | static inline void vegas_disable(struct sock *sk) |
b87d8561 | 102 | { |
6687e988 | 103 | struct vegas *vegas = inet_csk_ca(sk); |
b87d8561 SH |
104 | |
105 | vegas->doing_vegas_now = 0; | |
106 | } | |
107 | ||
6687e988 | 108 | static void tcp_vegas_init(struct sock *sk) |
b87d8561 | 109 | { |
6687e988 | 110 | struct vegas *vegas = inet_csk_ca(sk); |
b87d8561 SH |
111 | |
112 | vegas->baseRTT = 0x7fffffff; | |
6687e988 | 113 | vegas_enable(sk); |
b87d8561 SH |
114 | } |
115 | ||
116 | /* Do RTT sampling needed for Vegas. | |
117 | * Basically we: | |
118 | * o min-filter RTT samples from within an RTT to get the current | |
119 | * propagation delay + queuing delay (we are min-filtering to try to | |
120 | * avoid the effects of delayed ACKs) | |
121 | * o min-filter RTT samples from a much longer window (forever for now) | |
122 | * to find the propagation delay (baseRTT) | |
123 | */ | |
6687e988 | 124 | static void tcp_vegas_rtt_calc(struct sock *sk, u32 usrtt) |
b87d8561 | 125 | { |
6687e988 | 126 | struct vegas *vegas = inet_csk_ca(sk); |
b87d8561 SH |
127 | u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */ |
128 | ||
129 | /* Filter to find propagation delay: */ | |
130 | if (vrtt < vegas->baseRTT) | |
131 | vegas->baseRTT = vrtt; | |
132 | ||
133 | /* Find the min RTT during the last RTT to find | |
134 | * the current prop. delay + queuing delay: | |
135 | */ | |
136 | vegas->minRTT = min(vegas->minRTT, vrtt); | |
137 | vegas->cntRTT++; | |
138 | } | |
139 | ||
6687e988 | 140 | static void tcp_vegas_state(struct sock *sk, u8 ca_state) |
b87d8561 SH |
141 | { |
142 | ||
143 | if (ca_state == TCP_CA_Open) | |
6687e988 | 144 | vegas_enable(sk); |
b87d8561 | 145 | else |
6687e988 | 146 | vegas_disable(sk); |
b87d8561 SH |
147 | } |
148 | ||
149 | /* | |
150 | * If the connection is idle and we are restarting, | |
151 | * then we don't want to do any Vegas calculations | |
152 | * until we get fresh RTT samples. So when we | |
153 | * restart, we reset our Vegas state to a clean | |
154 | * slate. After we get acks for this flight of | |
155 | * packets, _then_ we can make Vegas calculations | |
156 | * again. | |
157 | */ | |
6687e988 | 158 | static void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event) |
b87d8561 SH |
159 | { |
160 | if (event == CA_EVENT_CWND_RESTART || | |
161 | event == CA_EVENT_TX_START) | |
6687e988 | 162 | tcp_vegas_init(sk); |
b87d8561 SH |
163 | } |
164 | ||
6687e988 | 165 | static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, |
b87d8561 SH |
166 | u32 seq_rtt, u32 in_flight, int flag) |
167 | { | |
6687e988 ACM |
168 | struct tcp_sock *tp = tcp_sk(sk); |
169 | struct vegas *vegas = inet_csk_ca(sk); | |
b87d8561 SH |
170 | |
171 | if (!vegas->doing_vegas_now) | |
6687e988 | 172 | return tcp_reno_cong_avoid(sk, ack, seq_rtt, in_flight, flag); |
b87d8561 SH |
173 | |
174 | /* The key players are v_beg_snd_una and v_beg_snd_nxt. | |
175 | * | |
176 | * These are so named because they represent the approximate values | |
177 | * of snd_una and snd_nxt at the beginning of the current RTT. More | |
178 | * precisely, they represent the amount of data sent during the RTT. | |
179 | * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt, | |
180 | * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding | |
181 | * bytes of data have been ACKed during the course of the RTT, giving | |
182 | * an "actual" rate of: | |
183 | * | |
184 | * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration) | |
185 | * | |
186 | * Unfortunately, v_beg_snd_una is not exactly equal to snd_una, | |
187 | * because delayed ACKs can cover more than one segment, so they | |
188 | * don't line up nicely with the boundaries of RTTs. | |
189 | * | |
190 | * Another unfortunate fact of life is that delayed ACKs delay the | |
191 | * advance of the left edge of our send window, so that the number | |
192 | * of bytes we send in an RTT is often less than our cwnd will allow. | |
193 | * So we keep track of our cwnd separately, in v_beg_snd_cwnd. | |
194 | */ | |
195 | ||
196 | if (after(ack, vegas->beg_snd_nxt)) { | |
197 | /* Do the Vegas once-per-RTT cwnd adjustment. */ | |
198 | u32 old_wnd, old_snd_cwnd; | |
199 | ||
200 | ||
201 | /* Here old_wnd is essentially the window of data that was | |
202 | * sent during the previous RTT, and has all | |
203 | * been acknowledged in the course of the RTT that ended | |
204 | * with the ACK we just received. Likewise, old_snd_cwnd | |
205 | * is the cwnd during the previous RTT. | |
206 | */ | |
207 | old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) / | |
208 | tp->mss_cache; | |
209 | old_snd_cwnd = vegas->beg_snd_cwnd; | |
210 | ||
211 | /* Save the extent of the current window so we can use this | |
212 | * at the end of the next RTT. | |
213 | */ | |
214 | vegas->beg_snd_una = vegas->beg_snd_nxt; | |
215 | vegas->beg_snd_nxt = tp->snd_nxt; | |
216 | vegas->beg_snd_cwnd = tp->snd_cwnd; | |
217 | ||
218 | /* Take into account the current RTT sample too, to | |
219 | * decrease the impact of delayed acks. This double counts | |
220 | * this sample since we count it for the next window as well, | |
221 | * but that's not too awful, since we're taking the min, | |
222 | * rather than averaging. | |
223 | */ | |
6687e988 | 224 | tcp_vegas_rtt_calc(sk, seq_rtt * 1000); |
b87d8561 SH |
225 | |
226 | /* We do the Vegas calculations only if we got enough RTT | |
227 | * samples that we can be reasonably sure that we got | |
228 | * at least one RTT sample that wasn't from a delayed ACK. | |
229 | * If we only had 2 samples total, | |
230 | * then that means we're getting only 1 ACK per RTT, which | |
231 | * means they're almost certainly delayed ACKs. | |
232 | * If we have 3 samples, we should be OK. | |
233 | */ | |
234 | ||
235 | if (vegas->cntRTT <= 2) { | |
236 | /* We don't have enough RTT samples to do the Vegas | |
237 | * calculation, so we'll behave like Reno. | |
238 | */ | |
239 | if (tp->snd_cwnd > tp->snd_ssthresh) | |
240 | tp->snd_cwnd++; | |
241 | } else { | |
242 | u32 rtt, target_cwnd, diff; | |
243 | ||
244 | /* We have enough RTT samples, so, using the Vegas | |
245 | * algorithm, we determine if we should increase or | |
246 | * decrease cwnd, and by how much. | |
247 | */ | |
248 | ||
249 | /* Pluck out the RTT we are using for the Vegas | |
250 | * calculations. This is the min RTT seen during the | |
251 | * last RTT. Taking the min filters out the effects | |
252 | * of delayed ACKs, at the cost of noticing congestion | |
253 | * a bit later. | |
254 | */ | |
255 | rtt = vegas->minRTT; | |
256 | ||
257 | /* Calculate the cwnd we should have, if we weren't | |
258 | * going too fast. | |
259 | * | |
260 | * This is: | |
261 | * (actual rate in segments) * baseRTT | |
262 | * We keep it as a fixed point number with | |
263 | * V_PARAM_SHIFT bits to the right of the binary point. | |
264 | */ | |
265 | target_cwnd = ((old_wnd * vegas->baseRTT) | |
266 | << V_PARAM_SHIFT) / rtt; | |
267 | ||
268 | /* Calculate the difference between the window we had, | |
269 | * and the window we would like to have. This quantity | |
270 | * is the "Diff" from the Arizona Vegas papers. | |
271 | * | |
272 | * Again, this is a fixed point number with | |
273 | * V_PARAM_SHIFT bits to the right of the binary | |
274 | * point. | |
275 | */ | |
276 | diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd; | |
277 | ||
278 | if (tp->snd_cwnd < tp->snd_ssthresh) { | |
279 | /* Slow start. */ | |
280 | if (diff > gamma) { | |
281 | /* Going too fast. Time to slow down | |
282 | * and switch to congestion avoidance. | |
283 | */ | |
284 | tp->snd_ssthresh = 2; | |
285 | ||
286 | /* Set cwnd to match the actual rate | |
287 | * exactly: | |
288 | * cwnd = (actual rate) * baseRTT | |
289 | * Then we add 1 because the integer | |
290 | * truncation robs us of full link | |
291 | * utilization. | |
292 | */ | |
293 | tp->snd_cwnd = min(tp->snd_cwnd, | |
294 | (target_cwnd >> | |
295 | V_PARAM_SHIFT)+1); | |
296 | ||
297 | } | |
298 | } else { | |
299 | /* Congestion avoidance. */ | |
300 | u32 next_snd_cwnd; | |
301 | ||
302 | /* Figure out where we would like cwnd | |
303 | * to be. | |
304 | */ | |
305 | if (diff > beta) { | |
306 | /* The old window was too fast, so | |
307 | * we slow down. | |
308 | */ | |
309 | next_snd_cwnd = old_snd_cwnd - 1; | |
310 | } else if (diff < alpha) { | |
311 | /* We don't have enough extra packets | |
312 | * in the network, so speed up. | |
313 | */ | |
314 | next_snd_cwnd = old_snd_cwnd + 1; | |
315 | } else { | |
316 | /* Sending just as fast as we | |
317 | * should be. | |
318 | */ | |
319 | next_snd_cwnd = old_snd_cwnd; | |
320 | } | |
321 | ||
322 | /* Adjust cwnd upward or downward, toward the | |
323 | * desired value. | |
324 | */ | |
325 | if (next_snd_cwnd > tp->snd_cwnd) | |
326 | tp->snd_cwnd++; | |
327 | else if (next_snd_cwnd < tp->snd_cwnd) | |
328 | tp->snd_cwnd--; | |
329 | } | |
330 | } | |
331 | ||
332 | /* Wipe the slate clean for the next RTT. */ | |
333 | vegas->cntRTT = 0; | |
334 | vegas->minRTT = 0x7fffffff; | |
335 | } | |
336 | ||
337 | /* The following code is executed for every ack we receive, | |
338 | * except for conditions checked in should_advance_cwnd() | |
339 | * before the call to tcp_cong_avoid(). Mainly this means that | |
340 | * we only execute this code if the ack actually acked some | |
341 | * data. | |
342 | */ | |
343 | ||
344 | /* If we are in slow start, increase our cwnd in response to this ACK. | |
345 | * (If we are not in slow start then we are in congestion avoidance, | |
346 | * and adjust our congestion window only once per RTT. See the code | |
347 | * above.) | |
348 | */ | |
349 | if (tp->snd_cwnd <= tp->snd_ssthresh) | |
350 | tp->snd_cwnd++; | |
351 | ||
352 | /* to keep cwnd from growing without bound */ | |
353 | tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp); | |
354 | ||
355 | /* Make sure that we are never so timid as to reduce our cwnd below | |
356 | * 2 MSS. | |
357 | * | |
358 | * Going below 2 MSS would risk huge delayed ACKs from our receiver. | |
359 | */ | |
360 | tp->snd_cwnd = max(tp->snd_cwnd, 2U); | |
361 | } | |
362 | ||
363 | /* Extract info for Tcp socket info provided via netlink. */ | |
6687e988 | 364 | static void tcp_vegas_get_info(struct sock *sk, u32 ext, |
b87d8561 SH |
365 | struct sk_buff *skb) |
366 | { | |
6687e988 | 367 | const struct vegas *ca = inet_csk_ca(sk); |
b87d8561 SH |
368 | if (ext & (1<<(TCPDIAG_VEGASINFO-1))) { |
369 | struct tcpvegas_info *info; | |
370 | ||
371 | info = RTA_DATA(__RTA_PUT(skb, TCPDIAG_VEGASINFO, | |
372 | sizeof(*info))); | |
373 | ||
374 | info->tcpv_enabled = ca->doing_vegas_now; | |
375 | info->tcpv_rttcnt = ca->cntRTT; | |
376 | info->tcpv_rtt = ca->baseRTT; | |
377 | info->tcpv_minrtt = ca->minRTT; | |
378 | rtattr_failure: ; | |
379 | } | |
380 | } | |
381 | ||
382 | static struct tcp_congestion_ops tcp_vegas = { | |
383 | .init = tcp_vegas_init, | |
384 | .ssthresh = tcp_reno_ssthresh, | |
385 | .cong_avoid = tcp_vegas_cong_avoid, | |
386 | .min_cwnd = tcp_reno_min_cwnd, | |
387 | .rtt_sample = tcp_vegas_rtt_calc, | |
388 | .set_state = tcp_vegas_state, | |
389 | .cwnd_event = tcp_vegas_cwnd_event, | |
390 | .get_info = tcp_vegas_get_info, | |
391 | ||
392 | .owner = THIS_MODULE, | |
393 | .name = "vegas", | |
394 | }; | |
395 | ||
396 | static int __init tcp_vegas_register(void) | |
397 | { | |
6687e988 | 398 | BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE); |
b87d8561 SH |
399 | tcp_register_congestion_control(&tcp_vegas); |
400 | return 0; | |
401 | } | |
402 | ||
403 | static void __exit tcp_vegas_unregister(void) | |
404 | { | |
405 | tcp_unregister_congestion_control(&tcp_vegas); | |
406 | } | |
407 | ||
408 | module_init(tcp_vegas_register); | |
409 | module_exit(tcp_vegas_unregister); | |
410 | ||
411 | MODULE_AUTHOR("Stephen Hemminger"); | |
412 | MODULE_LICENSE("GPL"); | |
413 | MODULE_DESCRIPTION("TCP Vegas"); |