1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * TCP Illinois congestion control.
4 * Home page:
5 *	http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
6 *
7 * The algorithm is described in:
8 * "TCP-Illinois: A Loss and Delay-Based Congestion Control Algorithm
9 *  for High-Speed Networks"
10 * http://tamerbasar.csl.illinois.edu/LiuBasarSrikantPerfEvalArtJun2008.pdf
11 *
12 * Implemented from description in paper and ns-2 simulation.
13 * Copyright (C) 2007 Stephen Hemminger <shemminger@linux-foundation.org>
14 */
15
16#include <linux/module.h>
17#include <linux/skbuff.h>
18#include <linux/inet_diag.h>
19#include <asm/div64.h>
20#include <net/tcp.h>
21
22#define ALPHA_SHIFT	7
23#define ALPHA_SCALE	(1u<<ALPHA_SHIFT)
24#define ALPHA_MIN	((3*ALPHA_SCALE)/10)	/* ~0.3 */
25#define ALPHA_MAX	(10*ALPHA_SCALE)	/* 10.0 */
26#define ALPHA_BASE	ALPHA_SCALE		/* 1.0 */
27#define RTT_MAX		(U32_MAX / ALPHA_MAX)	/* 3.3 secs */
28
29#define BETA_SHIFT	6
30#define BETA_SCALE	(1u<<BETA_SHIFT)
31#define BETA_MIN	(BETA_SCALE/8)		/* 0.125 */
32#define BETA_MAX	(BETA_SCALE/2)		/* 0.5 */
33#define BETA_BASE	BETA_MAX
34
35static int win_thresh __read_mostly = 15;
36module_param(win_thresh, int, 0);
37MODULE_PARM_DESC(win_thresh, "Window threshold for starting adaptive sizing");
38
39static int theta __read_mostly = 5;
40module_param(theta, int, 0);
41MODULE_PARM_DESC(theta, "# of fast RTT's before full growth");
42
43/* TCP Illinois Parameters */
44struct illinois {
45	u64	sum_rtt;	/* sum of rtt's measured within last rtt */
46	u16	cnt_rtt;	/* # of rtts measured within last rtt */
47	u32	base_rtt;	/* min of all rtt in usec */
48	u32	max_rtt;	/* max of all rtt in usec */
49	u32	end_seq;	/* right edge of current RTT */
50	u32	alpha;		/* Additive increase */
51	u32	beta;		/* Muliplicative decrease */
52	u16	acked;		/* # packets acked by current ACK */
53	u8	rtt_above;	/* average rtt has gone above threshold */
54	u8	rtt_low;	/* # of rtts measurements below threshold */
55};
56
57static void rtt_reset(struct sock *sk)
58{
59	struct tcp_sock *tp = tcp_sk(sk);
60	struct illinois *ca = inet_csk_ca(sk);
61
62	ca->end_seq = tp->snd_nxt;
63	ca->cnt_rtt = 0;
64	ca->sum_rtt = 0;
65
66	/* TODO: age max_rtt? */
67}
68
69static void tcp_illinois_init(struct sock *sk)
70{
71	struct illinois *ca = inet_csk_ca(sk);
72
73	ca->alpha = ALPHA_MAX;
74	ca->beta = BETA_BASE;
75	ca->base_rtt = 0x7fffffff;
76	ca->max_rtt = 0;
77
78	ca->acked = 0;
79	ca->rtt_low = 0;
80	ca->rtt_above = 0;
81
82	rtt_reset(sk);
83}
84
85/* Measure RTT for each ack. */
86static void tcp_illinois_acked(struct sock *sk, const struct ack_sample *sample)
87{
88	struct illinois *ca = inet_csk_ca(sk);
89	s32 rtt_us = sample->rtt_us;
90
91	ca->acked = sample->pkts_acked;
92
93	/* dup ack, no rtt sample */
94	if (rtt_us < 0)
95		return;
96
97	/* ignore bogus values, this prevents wraparound in alpha math */
98	if (rtt_us > RTT_MAX)
99		rtt_us = RTT_MAX;
100
101	/* keep track of minimum RTT seen so far */
102	if (ca->base_rtt > rtt_us)
103		ca->base_rtt = rtt_us;
104
105	/* and max */
106	if (ca->max_rtt < rtt_us)
107		ca->max_rtt = rtt_us;
108
109	++ca->cnt_rtt;
110	ca->sum_rtt += rtt_us;
111}
112
113/* Maximum queuing delay */
114static inline u32 max_delay(const struct illinois *ca)
115{
116	return ca->max_rtt - ca->base_rtt;
117}
118
119/* Average queuing delay */
120static inline u32 avg_delay(const struct illinois *ca)
121{
122	u64 t = ca->sum_rtt;
123
124	do_div(t, ca->cnt_rtt);
125	return t - ca->base_rtt;
126}
127
128/*
129 * Compute value of alpha used for additive increase.
130 * If small window then use 1.0, equivalent to Reno.
131 *
132 * For larger windows, adjust based on average delay.
133 * A. If average delay is at minimum (we are uncongested),
134 *    then use large alpha (10.0) to increase faster.
135 * B. If average delay is at maximum (getting congested)
136 *    then use small alpha (0.3)
137 *
138 * The result is a convex window growth curve.
139 */
140static u32 alpha(struct illinois *ca, u32 da, u32 dm)
141{
142	u32 d1 = dm / 100;	/* Low threshold */
143
144	if (da <= d1) {
145		/* If never got out of low delay zone, then use max */
146		if (!ca->rtt_above)
147			return ALPHA_MAX;
148
149		/* Wait for 5 good RTT's before allowing alpha to go alpha max.
150		 * This prevents one good RTT from causing sudden window increase.
151		 */
152		if (++ca->rtt_low < theta)
153			return ca->alpha;
154
155		ca->rtt_low = 0;
156		ca->rtt_above = 0;
157		return ALPHA_MAX;
158	}
159
160	ca->rtt_above = 1;
161
162	/*
163	 * Based on:
164	 *
165	 *      (dm - d1) amin amax
166	 * k1 = -------------------
167	 *         amax - amin
168	 *
169	 *       (dm - d1) amin
170	 * k2 = ----------------  - d1
171	 *        amax - amin
172	 *
173	 *             k1
174	 * alpha = ----------
175	 *          k2 + da
176	 */
177
178	dm -= d1;
179	da -= d1;
180	return (dm * ALPHA_MAX) /
181		(dm + (da  * (ALPHA_MAX - ALPHA_MIN)) / ALPHA_MIN);
182}
183
184/*
185 * Beta used for multiplicative decrease.
186 * For small window sizes returns same value as Reno (0.5)
187 *
188 * If delay is small (10% of max) then beta = 1/8
189 * If delay is up to 80% of max then beta = 1/2
190 * In between is a linear function
191 */
192static u32 beta(u32 da, u32 dm)
193{
194	u32 d2, d3;
195
196	d2 = dm / 10;
197	if (da <= d2)
198		return BETA_MIN;
199
200	d3 = (8 * dm) / 10;
201	if (da >= d3 || d3 <= d2)
202		return BETA_MAX;
203
204	/*
205	 * Based on:
206	 *
207	 *       bmin d3 - bmax d2
208	 * k3 = -------------------
209	 *         d3 - d2
210	 *
211	 *       bmax - bmin
212	 * k4 = -------------
213	 *         d3 - d2
214	 *
215	 * b = k3 + k4 da
216	 */
217	return (BETA_MIN * d3 - BETA_MAX * d2 + (BETA_MAX - BETA_MIN) * da)
218		/ (d3 - d2);
219}
220
221/* Update alpha and beta values once per RTT */
222static void update_params(struct sock *sk)
223{
224	struct tcp_sock *tp = tcp_sk(sk);
225	struct illinois *ca = inet_csk_ca(sk);
226
227	if (tcp_snd_cwnd(tp) < win_thresh) {
228		ca->alpha = ALPHA_BASE;
229		ca->beta = BETA_BASE;
230	} else if (ca->cnt_rtt > 0) {
231		u32 dm = max_delay(ca);
232		u32 da = avg_delay(ca);
233
234		ca->alpha = alpha(ca, da, dm);
235		ca->beta = beta(da, dm);
236	}
237
238	rtt_reset(sk);
239}
240
241/*
242 * In case of loss, reset to default values
243 */
244static void tcp_illinois_state(struct sock *sk, u8 new_state)
245{
246	struct illinois *ca = inet_csk_ca(sk);
247
248	if (new_state == TCP_CA_Loss) {
249		ca->alpha = ALPHA_BASE;
250		ca->beta = BETA_BASE;
251		ca->rtt_low = 0;
252		ca->rtt_above = 0;
253		rtt_reset(sk);
254	}
255}
256
257/*
258 * Increase window in response to successful acknowledgment.
259 */
260static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked)
261{
262	struct tcp_sock *tp = tcp_sk(sk);
263	struct illinois *ca = inet_csk_ca(sk);
264
265	if (after(ack, ca->end_seq))
266		update_params(sk);
267
268	/* RFC2861 only increase cwnd if fully utilized */
269	if (!tcp_is_cwnd_limited(sk))
270		return;
271
272	/* In slow start */
273	if (tcp_in_slow_start(tp))
274		tcp_slow_start(tp, acked);
275
276	else {
277		u32 delta;
278
279		/* snd_cwnd_cnt is # of packets since last cwnd increment */
280		tp->snd_cwnd_cnt += ca->acked;
281		ca->acked = 1;
282
283		/* This is close approximation of:
284		 * tp->snd_cwnd += alpha/tp->snd_cwnd
285		*/
286		delta = (tp->snd_cwnd_cnt * ca->alpha) >> ALPHA_SHIFT;
287		if (delta >= tcp_snd_cwnd(tp)) {
288			tcp_snd_cwnd_set(tp, min(tcp_snd_cwnd(tp) + delta / tcp_snd_cwnd(tp),
289						 (u32)tp->snd_cwnd_clamp));
290			tp->snd_cwnd_cnt = 0;
291		}
292	}
293}
294
295static u32 tcp_illinois_ssthresh(struct sock *sk)
296{
297	struct tcp_sock *tp = tcp_sk(sk);
298	struct illinois *ca = inet_csk_ca(sk);
299	u32 decr;
300
301	/* Multiplicative decrease */
302	decr = (tcp_snd_cwnd(tp) * ca->beta) >> BETA_SHIFT;
303	return max(tcp_snd_cwnd(tp) - decr, 2U);
304}
305
306/* Extract info for Tcp socket info provided via netlink. */
307static size_t tcp_illinois_info(struct sock *sk, u32 ext, int *attr,
308				union tcp_cc_info *info)
309{
310	const struct illinois *ca = inet_csk_ca(sk);
311
312	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
313		info->vegas.tcpv_enabled = 1;
314		info->vegas.tcpv_rttcnt = ca->cnt_rtt;
315		info->vegas.tcpv_minrtt = ca->base_rtt;
316		info->vegas.tcpv_rtt = 0;
317
318		if (info->vegas.tcpv_rttcnt > 0) {
319			u64 t = ca->sum_rtt;
320
321			do_div(t, info->vegas.tcpv_rttcnt);
322			info->vegas.tcpv_rtt = t;
323		}
324		*attr = INET_DIAG_VEGASINFO;
325		return sizeof(struct tcpvegas_info);
326	}
327	return 0;
328}
329
330static struct tcp_congestion_ops tcp_illinois __read_mostly = {
331	.init		= tcp_illinois_init,
332	.ssthresh	= tcp_illinois_ssthresh,
333	.undo_cwnd	= tcp_reno_undo_cwnd,
334	.cong_avoid	= tcp_illinois_cong_avoid,
335	.set_state	= tcp_illinois_state,
336	.get_info	= tcp_illinois_info,
337	.pkts_acked	= tcp_illinois_acked,
338
339	.owner		= THIS_MODULE,
340	.name		= "illinois",
341};
342
343static int __init tcp_illinois_register(void)
344{
345	BUILD_BUG_ON(sizeof(struct illinois) > ICSK_CA_PRIV_SIZE);
346	return tcp_register_congestion_control(&tcp_illinois);
347}
348
349static void __exit tcp_illinois_unregister(void)
350{
351	tcp_unregister_congestion_control(&tcp_illinois);
352}
353
354module_init(tcp_illinois_register);
355module_exit(tcp_illinois_unregister);
356
357MODULE_AUTHOR("Stephen Hemminger, Shao Liu");
358MODULE_LICENSE("GPL");
359MODULE_DESCRIPTION("TCP Illinois");
360MODULE_VERSION("1.0");
361