1/*
2 * TCP Illinois congestion control.
3 * Home page:
4 *	http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
5 *
6 * The algorithm is described in:
7 * "TCP-Illinois: A Loss and Delay-Based Congestion Control Algorithm
8 *  for High-Speed Networks"
9 * http://www.ifp.illinois.edu/~srikant/Papers/liubassri06perf.pdf
10 *
11 * Implemented from description in paper and ns-2 simulation.
12 * Copyright (C) 2007 Stephen Hemminger <shemminger@linux-foundation.org>
13 */
14
15#include <linux/module.h>
16#include <linux/skbuff.h>
17#include <linux/inet_diag.h>
18#include <asm/div64.h>
19#include <net/tcp.h>
20
21#define ALPHA_SHIFT	7
22#define ALPHA_SCALE	(1u<<ALPHA_SHIFT)
23#define ALPHA_MIN	((3*ALPHA_SCALE)/10)	/* ~0.3 */
24#define ALPHA_MAX	(10*ALPHA_SCALE)	/* 10.0 */
25#define ALPHA_BASE	ALPHA_SCALE		/* 1.0 */
26#define RTT_MAX		(U32_MAX / ALPHA_MAX)	/* 3.3 secs */
27
28#define BETA_SHIFT	6
29#define BETA_SCALE	(1u<<BETA_SHIFT)
30#define BETA_MIN	(BETA_SCALE/8)		/* 0.125 */
31#define BETA_MAX	(BETA_SCALE/2)		/* 0.5 */
32#define BETA_BASE	BETA_MAX
33
34static int win_thresh __read_mostly = 15;
35module_param(win_thresh, int, 0);
36MODULE_PARM_DESC(win_thresh, "Window threshold for starting adaptive sizing");
37
38static int theta __read_mostly = 5;
39module_param(theta, int, 0);
40MODULE_PARM_DESC(theta, "# of fast RTT's before full growth");
41
42/* TCP Illinois Parameters */
43struct illinois {
44	u64	sum_rtt;	/* sum of rtt's measured within last rtt */
45	u16	cnt_rtt;	/* # of rtts measured within last rtt */
46	u32	base_rtt;	/* min of all rtt in usec */
47	u32	max_rtt;	/* max of all rtt in usec */
48	u32	end_seq;	/* right edge of current RTT */
49	u32	alpha;		/* Additive increase */
50	u32	beta;		/* Muliplicative decrease */
51	u16	acked;		/* # packets acked by current ACK */
52	u8	rtt_above;	/* average rtt has gone above threshold */
53	u8	rtt_low;	/* # of rtts measurements below threshold */
54};
55
56static void rtt_reset(struct sock *sk)
57{
58	struct tcp_sock *tp = tcp_sk(sk);
59	struct illinois *ca = inet_csk_ca(sk);
60
61	ca->end_seq = tp->snd_nxt;
62	ca->cnt_rtt = 0;
63	ca->sum_rtt = 0;
64
65	/* TODO: age max_rtt? */
66}
67
68static void tcp_illinois_init(struct sock *sk)
69{
70	struct illinois *ca = inet_csk_ca(sk);
71
72	ca->alpha = ALPHA_MAX;
73	ca->beta = BETA_BASE;
74	ca->base_rtt = 0x7fffffff;
75	ca->max_rtt = 0;
76
77	ca->acked = 0;
78	ca->rtt_low = 0;
79	ca->rtt_above = 0;
80
81	rtt_reset(sk);
82}
83
84/* Measure RTT for each ack. */
85static void tcp_illinois_acked(struct sock *sk, u32 pkts_acked, s32 rtt)
86{
87	struct illinois *ca = inet_csk_ca(sk);
88
89	ca->acked = pkts_acked;
90
91	/* dup ack, no rtt sample */
92	if (rtt < 0)
93		return;
94
95	/* ignore bogus values, this prevents wraparound in alpha math */
96	if (rtt > RTT_MAX)
97		rtt = RTT_MAX;
98
99	/* keep track of minimum RTT seen so far */
100	if (ca->base_rtt > rtt)
101		ca->base_rtt = rtt;
102
103	/* and max */
104	if (ca->max_rtt < rtt)
105		ca->max_rtt = rtt;
106
107	++ca->cnt_rtt;
108	ca->sum_rtt += rtt;
109}
110
111/* Maximum queuing delay */
112static inline u32 max_delay(const struct illinois *ca)
113{
114	return ca->max_rtt - ca->base_rtt;
115}
116
117/* Average queuing delay */
118static inline u32 avg_delay(const struct illinois *ca)
119{
120	u64 t = ca->sum_rtt;
121
122	do_div(t, ca->cnt_rtt);
123	return t - ca->base_rtt;
124}
125
126/*
127 * Compute value of alpha used for additive increase.
128 * If small window then use 1.0, equivalent to Reno.
129 *
130 * For larger windows, adjust based on average delay.
131 * A. If average delay is at minimum (we are uncongested),
132 *    then use large alpha (10.0) to increase faster.
133 * B. If average delay is at maximum (getting congested)
134 *    then use small alpha (0.3)
135 *
136 * The result is a convex window growth curve.
137 */
138static u32 alpha(struct illinois *ca, u32 da, u32 dm)
139{
140	u32 d1 = dm / 100;	/* Low threshold */
141
142	if (da <= d1) {
143		/* If never got out of low delay zone, then use max */
144		if (!ca->rtt_above)
145			return ALPHA_MAX;
146
147		/* Wait for 5 good RTT's before allowing alpha to go alpha max.
148		 * This prevents one good RTT from causing sudden window increase.
149		 */
150		if (++ca->rtt_low < theta)
151			return ca->alpha;
152
153		ca->rtt_low = 0;
154		ca->rtt_above = 0;
155		return ALPHA_MAX;
156	}
157
158	ca->rtt_above = 1;
159
160	/*
161	 * Based on:
162	 *
163	 *      (dm - d1) amin amax
164	 * k1 = -------------------
165	 *         amax - amin
166	 *
167	 *       (dm - d1) amin
168	 * k2 = ----------------  - d1
169	 *        amax - amin
170	 *
171	 *             k1
172	 * alpha = ----------
173	 *          k2 + da
174	 */
175
176	dm -= d1;
177	da -= d1;
178	return (dm * ALPHA_MAX) /
179		(dm + (da  * (ALPHA_MAX - ALPHA_MIN)) / ALPHA_MIN);
180}
181
182/*
183 * Beta used for multiplicative decrease.
184 * For small window sizes returns same value as Reno (0.5)
185 *
186 * If delay is small (10% of max) then beta = 1/8
187 * If delay is up to 80% of max then beta = 1/2
188 * In between is a linear function
189 */
190static u32 beta(u32 da, u32 dm)
191{
192	u32 d2, d3;
193
194	d2 = dm / 10;
195	if (da <= d2)
196		return BETA_MIN;
197
198	d3 = (8 * dm) / 10;
199	if (da >= d3 || d3 <= d2)
200		return BETA_MAX;
201
202	/*
203	 * Based on:
204	 *
205	 *       bmin d3 - bmax d2
206	 * k3 = -------------------
207	 *         d3 - d2
208	 *
209	 *       bmax - bmin
210	 * k4 = -------------
211	 *         d3 - d2
212	 *
213	 * b = k3 + k4 da
214	 */
215	return (BETA_MIN * d3 - BETA_MAX * d2 + (BETA_MAX - BETA_MIN) * da)
216		/ (d3 - d2);
217}
218
219/* Update alpha and beta values once per RTT */
220static void update_params(struct sock *sk)
221{
222	struct tcp_sock *tp = tcp_sk(sk);
223	struct illinois *ca = inet_csk_ca(sk);
224
225	if (tp->snd_cwnd < win_thresh) {
226		ca->alpha = ALPHA_BASE;
227		ca->beta = BETA_BASE;
228	} else if (ca->cnt_rtt > 0) {
229		u32 dm = max_delay(ca);
230		u32 da = avg_delay(ca);
231
232		ca->alpha = alpha(ca, da, dm);
233		ca->beta = beta(da, dm);
234	}
235
236	rtt_reset(sk);
237}
238
239/*
240 * In case of loss, reset to default values
241 */
242static void tcp_illinois_state(struct sock *sk, u8 new_state)
243{
244	struct illinois *ca = inet_csk_ca(sk);
245
246	if (new_state == TCP_CA_Loss) {
247		ca->alpha = ALPHA_BASE;
248		ca->beta = BETA_BASE;
249		ca->rtt_low = 0;
250		ca->rtt_above = 0;
251		rtt_reset(sk);
252	}
253}
254
255/*
256 * Increase window in response to successful acknowledgment.
257 */
258static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked)
259{
260	struct tcp_sock *tp = tcp_sk(sk);
261	struct illinois *ca = inet_csk_ca(sk);
262
263	if (after(ack, ca->end_seq))
264		update_params(sk);
265
266	/* RFC2861 only increase cwnd if fully utilized */
267	if (!tcp_is_cwnd_limited(sk))
268		return;
269
270	/* In slow start */
271	if (tcp_in_slow_start(tp))
272		tcp_slow_start(tp, acked);
273
274	else {
275		u32 delta;
276
277		/* snd_cwnd_cnt is # of packets since last cwnd increment */
278		tp->snd_cwnd_cnt += ca->acked;
279		ca->acked = 1;
280
281		/* This is close approximation of:
282		 * tp->snd_cwnd += alpha/tp->snd_cwnd
283		*/
284		delta = (tp->snd_cwnd_cnt * ca->alpha) >> ALPHA_SHIFT;
285		if (delta >= tp->snd_cwnd) {
286			tp->snd_cwnd = min(tp->snd_cwnd + delta / tp->snd_cwnd,
287					   (u32)tp->snd_cwnd_clamp);
288			tp->snd_cwnd_cnt = 0;
289		}
290	}
291}
292
293static u32 tcp_illinois_ssthresh(struct sock *sk)
294{
295	struct tcp_sock *tp = tcp_sk(sk);
296	struct illinois *ca = inet_csk_ca(sk);
297
298	/* Multiplicative decrease */
299	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
300}
301
302/* Extract info for Tcp socket info provided via netlink. */
303static size_t tcp_illinois_info(struct sock *sk, u32 ext, int *attr,
304				union tcp_cc_info *info)
305{
306	const struct illinois *ca = inet_csk_ca(sk);
307
308	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
309		info->vegas.tcpv_enabled = 1;
310		info->vegas.tcpv_rttcnt = ca->cnt_rtt;
311		info->vegas.tcpv_minrtt = ca->base_rtt;
312		info->vegas.tcpv_rtt = 0;
313
314		if (info->vegas.tcpv_rttcnt > 0) {
315			u64 t = ca->sum_rtt;
316
317			do_div(t, info->vegas.tcpv_rttcnt);
318			info->vegas.tcpv_rtt = t;
319		}
320		*attr = INET_DIAG_VEGASINFO;
321		return sizeof(struct tcpvegas_info);
322	}
323	return 0;
324}
325
326static struct tcp_congestion_ops tcp_illinois __read_mostly = {
327	.init		= tcp_illinois_init,
328	.ssthresh	= tcp_illinois_ssthresh,
329	.cong_avoid	= tcp_illinois_cong_avoid,
330	.set_state	= tcp_illinois_state,
331	.get_info	= tcp_illinois_info,
332	.pkts_acked	= tcp_illinois_acked,
333
334	.owner		= THIS_MODULE,
335	.name		= "illinois",
336};
337
338static int __init tcp_illinois_register(void)
339{
340	BUILD_BUG_ON(sizeof(struct illinois) > ICSK_CA_PRIV_SIZE);
341	return tcp_register_congestion_control(&tcp_illinois);
342}
343
344static void __exit tcp_illinois_unregister(void)
345{
346	tcp_unregister_congestion_control(&tcp_illinois);
347}
348
349module_init(tcp_illinois_register);
350module_exit(tcp_illinois_unregister);
351
352MODULE_AUTHOR("Stephen Hemminger, Shao Liu");
353MODULE_LICENSE("GPL");
354MODULE_DESCRIPTION("TCP Illinois");
355MODULE_VERSION("1.0");
356