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Linux/net/ipv4/tcp_vegas.c

  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 /* 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  */
 70 static 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. */
 86 static 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 
 93 void 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 }
100 EXPORT_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  */
110 void 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 }
131 EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);
132 
133 void 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 }
140 EXPORT_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  */
151 void 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 }
157 EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
158 
159 static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
160 {
161         return  min(tp->snd_ssthresh, tp->snd_cwnd-1);
162 }
163 
164 static 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. */
289 size_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 }
305 EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
306 
307 static struct tcp_congestion_ops tcp_vegas __read_mostly = {
308         .init           = tcp_vegas_init,
309         .ssthresh       = tcp_reno_ssthresh,
310         .cong_avoid     = tcp_vegas_cong_avoid,
311         .pkts_acked     = tcp_vegas_pkts_acked,
312         .set_state      = tcp_vegas_state,
313         .cwnd_event     = tcp_vegas_cwnd_event,
314         .get_info       = tcp_vegas_get_info,
315 
316         .owner          = THIS_MODULE,
317         .name           = "vegas",
318 };
319 
320 static int __init tcp_vegas_register(void)
321 {
322         BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
323         tcp_register_congestion_control(&tcp_vegas);
324         return 0;
325 }
326 
327 static void __exit tcp_vegas_unregister(void)
328 {
329         tcp_unregister_congestion_control(&tcp_vegas);
330 }
331 
332 module_init(tcp_vegas_register);
333 module_exit(tcp_vegas_unregister);
334 
335 MODULE_AUTHOR("Stephen Hemminger");
336 MODULE_LICENSE("GPL");
337 MODULE_DESCRIPTION("TCP Vegas");
338 

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