1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * TCP Vegas congestion control
4 *
5 * This is based on the congestion detection/avoidance scheme described in
6 *    Lawrence S. Brakmo and Larry L. Peterson.
7 *    "TCP Vegas: End to end congestion avoidance on a global internet."
8 *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
9 *    October 1995. Available from:
10 *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
11 *
12 * See http://www.cs.arizona.edu/xkernel/ for their implementation.
13 * The main aspects that distinguish this implementation from the
14 * Arizona Vegas implementation are:
15 *   o We do not change the loss detection or recovery mechanisms of
16 *     Linux in any way. Linux already recovers from losses quite well,
17 *     using fine-grained timers, NewReno, and FACK.
18 *   o To avoid the performance penalty imposed by increasing cwnd
19 *     only every-other RTT during slow start, we increase during
20 *     every RTT during slow start, just like Reno.
21 *   o Largely to allow continuous cwnd growth during slow start,
22 *     we use the rate at which ACKs come back as the "actual"
23 *     rate, rather than the rate at which data is sent.
24 *   o To speed convergence to the right rate, we set the cwnd
25 *     to achieve the right ("actual") rate when we exit slow start.
26 *   o To filter out the noise caused by delayed ACKs, we use the
27 *     minimum RTT sample observed during the last RTT to calculate
28 *     the actual rate.
29 *   o When the sender re-starts from idle, it waits until it has
30 *     received ACKs for an entire flight of new data before making
31 *     a cwnd adjustment decision. The original Vegas implementation
32 *     assumed senders never went idle.
33 */
34
35#include <linux/mm.h>
36#include <linux/module.h>
37#include <linux/skbuff.h>
38#include <linux/inet_diag.h>
39
40#include <net/tcp.h>
41
42#include "tcp_vegas.h"
43
44static int alpha = 2;
45static int beta  = 4;
46static int gamma = 1;
47
48module_param(alpha, int, 0644);
49MODULE_PARM_DESC(alpha, "lower bound of packets in network");
50module_param(beta, int, 0644);
51MODULE_PARM_DESC(beta, "upper bound of packets in network");
52module_param(gamma, int, 0644);
53MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
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 */
71static 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. */
87static 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
94void 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}
101EXPORT_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 */
111void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample)
112{
113	struct vegas *vegas = inet_csk_ca(sk);
114	u32 vrtt;
115
116	if (sample->rtt_us < 0)
117		return;
118
119	/* Never allow zero rtt or baseRTT */
120	vrtt = sample->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}
132EXPORT_SYMBOL_GPL(tcp_vegas_pkts_acked);
133
134void tcp_vegas_state(struct sock *sk, u8 ca_state)
135{
136	if (ca_state == TCP_CA_Open)
137		vegas_enable(sk);
138	else
139		vegas_disable(sk);
140}
141EXPORT_SYMBOL_GPL(tcp_vegas_state);
142
143/*
144 * If the connection is idle and we are restarting,
145 * then we don't want to do any Vegas calculations
146 * until we get fresh RTT samples.  So when we
147 * restart, we reset our Vegas state to a clean
148 * slate. After we get acks for this flight of
149 * packets, _then_ we can make Vegas calculations
150 * again.
151 */
152void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
153{
154	if (event == CA_EVENT_CWND_RESTART ||
155	    event == CA_EVENT_TX_START)
156		tcp_vegas_init(sk);
157}
158EXPORT_SYMBOL_GPL(tcp_vegas_cwnd_event);
159
160static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
161{
162	return  min(tp->snd_ssthresh, tcp_snd_cwnd(tp));
163}
164
165static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
166{
167	struct tcp_sock *tp = tcp_sk(sk);
168	struct vegas *vegas = inet_csk_ca(sk);
169
170	if (!vegas->doing_vegas_now) {
171		tcp_reno_cong_avoid(sk, ack, acked);
172		return;
173	}
174
175	if (after(ack, vegas->beg_snd_nxt)) {
176		/* Do the Vegas once-per-RTT cwnd adjustment. */
177
178		/* Save the extent of the current window so we can use this
179		 * at the end of the next RTT.
180		 */
181		vegas->beg_snd_nxt  = tp->snd_nxt;
182
183		/* We do the Vegas calculations only if we got enough RTT
184		 * samples that we can be reasonably sure that we got
185		 * at least one RTT sample that wasn't from a delayed ACK.
186		 * If we only had 2 samples total,
187		 * then that means we're getting only 1 ACK per RTT, which
188		 * means they're almost certainly delayed ACKs.
189		 * If  we have 3 samples, we should be OK.
190		 */
191
192		if (vegas->cntRTT <= 2) {
193			/* We don't have enough RTT samples to do the Vegas
194			 * calculation, so we'll behave like Reno.
195			 */
196			tcp_reno_cong_avoid(sk, ack, acked);
197		} else {
198			u32 rtt, diff;
199			u64 target_cwnd;
200
201			/* We have enough RTT samples, so, using the Vegas
202			 * algorithm, we determine if we should increase or
203			 * decrease cwnd, and by how much.
204			 */
205
206			/* Pluck out the RTT we are using for the Vegas
207			 * calculations. This is the min RTT seen during the
208			 * last RTT. Taking the min filters out the effects
209			 * of delayed ACKs, at the cost of noticing congestion
210			 * a bit later.
211			 */
212			rtt = vegas->minRTT;
213
214			/* Calculate the cwnd we should have, if we weren't
215			 * going too fast.
216			 *
217			 * This is:
218			 *     (actual rate in segments) * baseRTT
219			 */
220			target_cwnd = (u64)tcp_snd_cwnd(tp) * vegas->baseRTT;
221			do_div(target_cwnd, rtt);
222
223			/* Calculate the difference between the window we had,
224			 * and the window we would like to have. This quantity
225			 * is the "Diff" from the Arizona Vegas papers.
226			 */
227			diff = tcp_snd_cwnd(tp) * (rtt-vegas->baseRTT) / vegas->baseRTT;
228
229			if (diff > gamma && tcp_in_slow_start(tp)) {
230				/* Going too fast. Time to slow down
231				 * and switch to congestion avoidance.
232				 */
233
234				/* Set cwnd to match the actual rate
235				 * exactly:
236				 *   cwnd = (actual rate) * baseRTT
237				 * Then we add 1 because the integer
238				 * truncation robs us of full link
239				 * utilization.
240				 */
241				tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp),
242							 (u32)target_cwnd + 1));
243				tp->snd_ssthresh = tcp_vegas_ssthresh(tp);
244
245			} else if (tcp_in_slow_start(tp)) {
246				/* Slow start.  */
247				tcp_slow_start(tp, acked);
248			} else {
249				/* Congestion avoidance. */
250
251				/* Figure out where we would like cwnd
252				 * to be.
253				 */
254				if (diff > beta) {
255					/* The old window was too fast, so
256					 * we slow down.
257					 */
258					tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
259					tp->snd_ssthresh
260						= tcp_vegas_ssthresh(tp);
261				} else if (diff < alpha) {
262					/* We don't have enough extra packets
263					 * in the network, so speed up.
264					 */
265					tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
266				} else {
267					/* Sending just as fast as we
268					 * should be.
269					 */
270				}
271			}
272
273			if (tcp_snd_cwnd(tp) < 2)
274				tcp_snd_cwnd_set(tp, 2);
275			else if (tcp_snd_cwnd(tp) > tp->snd_cwnd_clamp)
276				tcp_snd_cwnd_set(tp, tp->snd_cwnd_clamp);
277
278			tp->snd_ssthresh = tcp_current_ssthresh(sk);
279		}
280
281		/* Wipe the slate clean for the next RTT. */
282		vegas->cntRTT = 0;
283		vegas->minRTT = 0x7fffffff;
284	}
285	/* Use normal slow start */
286	else if (tcp_in_slow_start(tp))
287		tcp_slow_start(tp, acked);
288}
289
290/* Extract info for Tcp socket info provided via netlink. */
291size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
292			  union tcp_cc_info *info)
293{
294	const struct vegas *ca = inet_csk_ca(sk);
295
296	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
297		info->vegas.tcpv_enabled = ca->doing_vegas_now;
298		info->vegas.tcpv_rttcnt = ca->cntRTT;
299		info->vegas.tcpv_rtt = ca->baseRTT;
300		info->vegas.tcpv_minrtt = ca->minRTT;
301
302		*attr = INET_DIAG_VEGASINFO;
303		return sizeof(struct tcpvegas_info);
304	}
305	return 0;
306}
307EXPORT_SYMBOL_GPL(tcp_vegas_get_info);
308
309static struct tcp_congestion_ops tcp_vegas __read_mostly = {
310	.init		= tcp_vegas_init,
311	.ssthresh	= tcp_reno_ssthresh,
312	.undo_cwnd	= tcp_reno_undo_cwnd,
313	.cong_avoid	= tcp_vegas_cong_avoid,
314	.pkts_acked	= tcp_vegas_pkts_acked,
315	.set_state	= tcp_vegas_state,
316	.cwnd_event	= tcp_vegas_cwnd_event,
317	.get_info	= tcp_vegas_get_info,
318
319	.owner		= THIS_MODULE,
320	.name		= "vegas",
321};
322
323static int __init tcp_vegas_register(void)
324{
325	BUILD_BUG_ON(sizeof(struct vegas) > ICSK_CA_PRIV_SIZE);
326	tcp_register_congestion_control(&tcp_vegas);
327	return 0;
328}
329
330static void __exit tcp_vegas_unregister(void)
331{
332	tcp_unregister_congestion_control(&tcp_vegas);
333}
334
335module_init(tcp_vegas_register);
336module_exit(tcp_vegas_unregister);
337
338MODULE_AUTHOR("Stephen Hemminger");
339MODULE_LICENSE("GPL");
340MODULE_DESCRIPTION("TCP Vegas");
341