2 * TCP Vegas congestion control
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
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
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.
34 #include <linux/config.h>
36 #include <linux/module.h>
37 #include <linux/skbuff.h>
38 #include <linux/tcp_diag.h>
42 /* Default values of the Vegas variables, in fixed-point representation
43 * with V_PARAM_SHIFT bits to the right of the binary point.
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
;
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)");
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) */
69 /* There are several situations when we must "re-start" Vegas:
71 * o when a connection is established
73 * o after fast recovery
74 * o when we send a packet and there is no outstanding
75 * unacknowledged data (restarting an idle connection)
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
82 * Instead we must wait until the completion of an RTT during
83 * which we actually receive ACKs.
85 static inline void vegas_enable(struct tcp_sock
*tp
)
87 struct vegas
*vegas
= tcp_ca(tp
);
89 /* Begin taking Vegas samples next time we send something. */
90 vegas
->doing_vegas_now
= 1;
92 /* Set the beginning of the next send window. */
93 vegas
->beg_snd_nxt
= tp
->snd_nxt
;
96 vegas
->minRTT
= 0x7fffffff;
99 /* Stop taking Vegas samples for now. */
100 static inline void vegas_disable(struct tcp_sock
*tp
)
102 struct vegas
*vegas
= tcp_ca(tp
);
104 vegas
->doing_vegas_now
= 0;
107 static void tcp_vegas_init(struct tcp_sock
*tp
)
109 struct vegas
*vegas
= tcp_ca(tp
);
111 vegas
->baseRTT
= 0x7fffffff;
115 /* Do RTT sampling needed for Vegas.
117 * o min-filter RTT samples from within an RTT to get the current
118 * propagation delay + queuing delay (we are min-filtering to try to
119 * avoid the effects of delayed ACKs)
120 * o min-filter RTT samples from a much longer window (forever for now)
121 * to find the propagation delay (baseRTT)
123 static void tcp_vegas_rtt_calc(struct tcp_sock
*tp
, u32 usrtt
)
125 struct vegas
*vegas
= tcp_ca(tp
);
126 u32 vrtt
= usrtt
+ 1; /* Never allow zero rtt or baseRTT */
128 /* Filter to find propagation delay: */
129 if (vrtt
< vegas
->baseRTT
)
130 vegas
->baseRTT
= vrtt
;
132 /* Find the min RTT during the last RTT to find
133 * the current prop. delay + queuing delay:
135 vegas
->minRTT
= min(vegas
->minRTT
, vrtt
);
139 static void tcp_vegas_state(struct tcp_sock
*tp
, u8 ca_state
)
142 if (ca_state
== TCP_CA_Open
)
149 * If the connection is idle and we are restarting,
150 * then we don't want to do any Vegas calculations
151 * until we get fresh RTT samples. So when we
152 * restart, we reset our Vegas state to a clean
153 * slate. After we get acks for this flight of
154 * packets, _then_ we can make Vegas calculations
157 static void tcp_vegas_cwnd_event(struct tcp_sock
*tp
, enum tcp_ca_event event
)
159 if (event
== CA_EVENT_CWND_RESTART
||
160 event
== CA_EVENT_TX_START
)
164 static void tcp_vegas_cong_avoid(struct tcp_sock
*tp
, u32 ack
,
165 u32 seq_rtt
, u32 in_flight
, int flag
)
167 struct vegas
*vegas
= tcp_ca(tp
);
169 if (!vegas
->doing_vegas_now
)
170 return tcp_reno_cong_avoid(tp
, ack
, seq_rtt
, in_flight
, flag
);
172 /* The key players are v_beg_snd_una and v_beg_snd_nxt.
174 * These are so named because they represent the approximate values
175 * of snd_una and snd_nxt at the beginning of the current RTT. More
176 * precisely, they represent the amount of data sent during the RTT.
177 * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
178 * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
179 * bytes of data have been ACKed during the course of the RTT, giving
180 * an "actual" rate of:
182 * (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
184 * Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
185 * because delayed ACKs can cover more than one segment, so they
186 * don't line up nicely with the boundaries of RTTs.
188 * Another unfortunate fact of life is that delayed ACKs delay the
189 * advance of the left edge of our send window, so that the number
190 * of bytes we send in an RTT is often less than our cwnd will allow.
191 * So we keep track of our cwnd separately, in v_beg_snd_cwnd.
194 if (after(ack
, vegas
->beg_snd_nxt
)) {
195 /* Do the Vegas once-per-RTT cwnd adjustment. */
196 u32 old_wnd
, old_snd_cwnd
;
199 /* Here old_wnd is essentially the window of data that was
200 * sent during the previous RTT, and has all
201 * been acknowledged in the course of the RTT that ended
202 * with the ACK we just received. Likewise, old_snd_cwnd
203 * is the cwnd during the previous RTT.
205 old_wnd
= (vegas
->beg_snd_nxt
- vegas
->beg_snd_una
) /
207 old_snd_cwnd
= vegas
->beg_snd_cwnd
;
209 /* Save the extent of the current window so we can use this
210 * at the end of the next RTT.
212 vegas
->beg_snd_una
= vegas
->beg_snd_nxt
;
213 vegas
->beg_snd_nxt
= tp
->snd_nxt
;
214 vegas
->beg_snd_cwnd
= tp
->snd_cwnd
;
216 /* Take into account the current RTT sample too, to
217 * decrease the impact of delayed acks. This double counts
218 * this sample since we count it for the next window as well,
219 * but that's not too awful, since we're taking the min,
220 * rather than averaging.
222 tcp_vegas_rtt_calc(tp
, seq_rtt
*1000);
224 /* We do the Vegas calculations only if we got enough RTT
225 * samples that we can be reasonably sure that we got
226 * at least one RTT sample that wasn't from a delayed ACK.
227 * If we only had 2 samples total,
228 * then that means we're getting only 1 ACK per RTT, which
229 * means they're almost certainly delayed ACKs.
230 * If we have 3 samples, we should be OK.
233 if (vegas
->cntRTT
<= 2) {
234 /* We don't have enough RTT samples to do the Vegas
235 * calculation, so we'll behave like Reno.
237 if (tp
->snd_cwnd
> tp
->snd_ssthresh
)
240 u32 rtt
, target_cwnd
, diff
;
242 /* We have enough RTT samples, so, using the Vegas
243 * algorithm, we determine if we should increase or
244 * decrease cwnd, and by how much.
247 /* Pluck out the RTT we are using for the Vegas
248 * calculations. This is the min RTT seen during the
249 * last RTT. Taking the min filters out the effects
250 * of delayed ACKs, at the cost of noticing congestion
255 /* Calculate the cwnd we should have, if we weren't
259 * (actual rate in segments) * baseRTT
260 * We keep it as a fixed point number with
261 * V_PARAM_SHIFT bits to the right of the binary point.
263 target_cwnd
= ((old_wnd
* vegas
->baseRTT
)
264 << V_PARAM_SHIFT
) / rtt
;
266 /* Calculate the difference between the window we had,
267 * and the window we would like to have. This quantity
268 * is the "Diff" from the Arizona Vegas papers.
270 * Again, this is a fixed point number with
271 * V_PARAM_SHIFT bits to the right of the binary
274 diff
= (old_wnd
<< V_PARAM_SHIFT
) - target_cwnd
;
276 if (tp
->snd_cwnd
< tp
->snd_ssthresh
) {
279 /* Going too fast. Time to slow down
280 * and switch to congestion avoidance.
282 tp
->snd_ssthresh
= 2;
284 /* Set cwnd to match the actual rate
286 * cwnd = (actual rate) * baseRTT
287 * Then we add 1 because the integer
288 * truncation robs us of full link
291 tp
->snd_cwnd
= min(tp
->snd_cwnd
,
297 /* Congestion avoidance. */
300 /* Figure out where we would like cwnd
304 /* The old window was too fast, so
307 next_snd_cwnd
= old_snd_cwnd
- 1;
308 } else if (diff
< alpha
) {
309 /* We don't have enough extra packets
310 * in the network, so speed up.
312 next_snd_cwnd
= old_snd_cwnd
+ 1;
314 /* Sending just as fast as we
317 next_snd_cwnd
= old_snd_cwnd
;
320 /* Adjust cwnd upward or downward, toward the
323 if (next_snd_cwnd
> tp
->snd_cwnd
)
325 else if (next_snd_cwnd
< tp
->snd_cwnd
)
330 /* Wipe the slate clean for the next RTT. */
332 vegas
->minRTT
= 0x7fffffff;
335 /* The following code is executed for every ack we receive,
336 * except for conditions checked in should_advance_cwnd()
337 * before the call to tcp_cong_avoid(). Mainly this means that
338 * we only execute this code if the ack actually acked some
342 /* If we are in slow start, increase our cwnd in response to this ACK.
343 * (If we are not in slow start then we are in congestion avoidance,
344 * and adjust our congestion window only once per RTT. See the code
347 if (tp
->snd_cwnd
<= tp
->snd_ssthresh
)
350 /* to keep cwnd from growing without bound */
351 tp
->snd_cwnd
= min_t(u32
, tp
->snd_cwnd
, tp
->snd_cwnd_clamp
);
353 /* Make sure that we are never so timid as to reduce our cwnd below
356 * Going below 2 MSS would risk huge delayed ACKs from our receiver.
358 tp
->snd_cwnd
= max(tp
->snd_cwnd
, 2U);
361 /* Extract info for Tcp socket info provided via netlink. */
362 static void tcp_vegas_get_info(struct tcp_sock
*tp
, u32 ext
,
365 const struct vegas
*ca
= tcp_ca(tp
);
366 if (ext
& (1<<(TCPDIAG_VEGASINFO
-1))) {
367 struct tcpvegas_info
*info
;
369 info
= RTA_DATA(__RTA_PUT(skb
, TCPDIAG_VEGASINFO
,
372 info
->tcpv_enabled
= ca
->doing_vegas_now
;
373 info
->tcpv_rttcnt
= ca
->cntRTT
;
374 info
->tcpv_rtt
= ca
->baseRTT
;
375 info
->tcpv_minrtt
= ca
->minRTT
;
380 static struct tcp_congestion_ops tcp_vegas
= {
381 .init
= tcp_vegas_init
,
382 .ssthresh
= tcp_reno_ssthresh
,
383 .cong_avoid
= tcp_vegas_cong_avoid
,
384 .min_cwnd
= tcp_reno_min_cwnd
,
385 .rtt_sample
= tcp_vegas_rtt_calc
,
386 .set_state
= tcp_vegas_state
,
387 .cwnd_event
= tcp_vegas_cwnd_event
,
388 .get_info
= tcp_vegas_get_info
,
390 .owner
= THIS_MODULE
,
394 static int __init
tcp_vegas_register(void)
396 BUG_ON(sizeof(struct vegas
) > TCP_CA_PRIV_SIZE
);
397 tcp_register_congestion_control(&tcp_vegas
);
401 static void __exit
tcp_vegas_unregister(void)
403 tcp_unregister_congestion_control(&tcp_vegas
);
406 module_init(tcp_vegas_register
);
407 module_exit(tcp_vegas_unregister
);
409 MODULE_AUTHOR("Stephen Hemminger");
410 MODULE_LICENSE("GPL");
411 MODULE_DESCRIPTION("TCP Vegas");