1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Sally Floyd's High Speed TCP (RFC 3649) congestion control
4 *
5 * See https://www.icir.org/floyd/hstcp.html
6 *
7 * John Heffner <jheffner@psc.edu>
8 */
9
10#include <linux/module.h>
11#include <net/tcp.h>
12
13/* From AIMD tables from RFC 3649 appendix B,
14 * with fixed-point MD scaled <<8.
15 */
16static const struct hstcp_aimd_val {
17	unsigned int cwnd;
18	unsigned int md;
19} hstcp_aimd_vals[] = {
20	{     38,  128, /*  0.50 */ },
21	{    118,  112, /*  0.44 */ },
22	{    221,  104, /*  0.41 */ },
23	{    347,   98, /*  0.38 */ },
24	{    495,   93, /*  0.37 */ },
25	{    663,   89, /*  0.35 */ },
26	{    851,   86, /*  0.34 */ },
27	{   1058,   83, /*  0.33 */ },
28	{   1284,   81, /*  0.32 */ },
29	{   1529,   78, /*  0.31 */ },
30	{   1793,   76, /*  0.30 */ },
31	{   2076,   74, /*  0.29 */ },
32	{   2378,   72, /*  0.28 */ },
33	{   2699,   71, /*  0.28 */ },
34	{   3039,   69, /*  0.27 */ },
35	{   3399,   68, /*  0.27 */ },
36	{   3778,   66, /*  0.26 */ },
37	{   4177,   65, /*  0.26 */ },
38	{   4596,   64, /*  0.25 */ },
39	{   5036,   62, /*  0.25 */ },
40	{   5497,   61, /*  0.24 */ },
41	{   5979,   60, /*  0.24 */ },
42	{   6483,   59, /*  0.23 */ },
43	{   7009,   58, /*  0.23 */ },
44	{   7558,   57, /*  0.22 */ },
45	{   8130,   56, /*  0.22 */ },
46	{   8726,   55, /*  0.22 */ },
47	{   9346,   54, /*  0.21 */ },
48	{   9991,   53, /*  0.21 */ },
49	{  10661,   52, /*  0.21 */ },
50	{  11358,   52, /*  0.20 */ },
51	{  12082,   51, /*  0.20 */ },
52	{  12834,   50, /*  0.20 */ },
53	{  13614,   49, /*  0.19 */ },
54	{  14424,   48, /*  0.19 */ },
55	{  15265,   48, /*  0.19 */ },
56	{  16137,   47, /*  0.19 */ },
57	{  17042,   46, /*  0.18 */ },
58	{  17981,   45, /*  0.18 */ },
59	{  18955,   45, /*  0.18 */ },
60	{  19965,   44, /*  0.17 */ },
61	{  21013,   43, /*  0.17 */ },
62	{  22101,   43, /*  0.17 */ },
63	{  23230,   42, /*  0.17 */ },
64	{  24402,   41, /*  0.16 */ },
65	{  25618,   41, /*  0.16 */ },
66	{  26881,   40, /*  0.16 */ },
67	{  28193,   39, /*  0.16 */ },
68	{  29557,   39, /*  0.15 */ },
69	{  30975,   38, /*  0.15 */ },
70	{  32450,   38, /*  0.15 */ },
71	{  33986,   37, /*  0.15 */ },
72	{  35586,   36, /*  0.14 */ },
73	{  37253,   36, /*  0.14 */ },
74	{  38992,   35, /*  0.14 */ },
75	{  40808,   35, /*  0.14 */ },
76	{  42707,   34, /*  0.13 */ },
77	{  44694,   33, /*  0.13 */ },
78	{  46776,   33, /*  0.13 */ },
79	{  48961,   32, /*  0.13 */ },
80	{  51258,   32, /*  0.13 */ },
81	{  53677,   31, /*  0.12 */ },
82	{  56230,   30, /*  0.12 */ },
83	{  58932,   30, /*  0.12 */ },
84	{  61799,   29, /*  0.12 */ },
85	{  64851,   28, /*  0.11 */ },
86	{  68113,   28, /*  0.11 */ },
87	{  71617,   27, /*  0.11 */ },
88	{  75401,   26, /*  0.10 */ },
89	{  79517,   26, /*  0.10 */ },
90	{  84035,   25, /*  0.10 */ },
91	{  89053,   24, /*  0.10 */ },
92};
93
94#define HSTCP_AIMD_MAX	ARRAY_SIZE(hstcp_aimd_vals)
95
96struct hstcp {
97	u32	ai;
98};
99
100static void hstcp_init(struct sock *sk)
101{
102	struct tcp_sock *tp = tcp_sk(sk);
103	struct hstcp *ca = inet_csk_ca(sk);
104
105	ca->ai = 0;
106
107	/* Ensure the MD arithmetic works.  This is somewhat pedantic,
108	 * since I don't think we will see a cwnd this large. :) */
109	tp->snd_cwnd_clamp = min_t(u32, tp->snd_cwnd_clamp, 0xffffffff/128);
110}
111
112static void hstcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
113{
114	struct tcp_sock *tp = tcp_sk(sk);
115	struct hstcp *ca = inet_csk_ca(sk);
116
117	if (!tcp_is_cwnd_limited(sk))
118		return;
119
120	if (tcp_in_slow_start(tp))
121		tcp_slow_start(tp, acked);
122	else {
123		/* Update AIMD parameters.
124		 *
125		 * We want to guarantee that:
126		 *     hstcp_aimd_vals[ca->ai-1].cwnd <
127		 *     snd_cwnd <=
128		 *     hstcp_aimd_vals[ca->ai].cwnd
129		 */
130		if (tcp_snd_cwnd(tp) > hstcp_aimd_vals[ca->ai].cwnd) {
131			while (tcp_snd_cwnd(tp) > hstcp_aimd_vals[ca->ai].cwnd &&
132			       ca->ai < HSTCP_AIMD_MAX - 1)
133				ca->ai++;
134		} else if (ca->ai && tcp_snd_cwnd(tp) <= hstcp_aimd_vals[ca->ai-1].cwnd) {
135			while (ca->ai && tcp_snd_cwnd(tp) <= hstcp_aimd_vals[ca->ai-1].cwnd)
136				ca->ai--;
137		}
138
139		/* Do additive increase */
140		if (tcp_snd_cwnd(tp) < tp->snd_cwnd_clamp) {
141			/* cwnd = cwnd + a(w) / cwnd */
142			tp->snd_cwnd_cnt += ca->ai + 1;
143			if (tp->snd_cwnd_cnt >= tcp_snd_cwnd(tp)) {
144				tp->snd_cwnd_cnt -= tcp_snd_cwnd(tp);
145				tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) + 1);
146			}
147		}
148	}
149}
150
151static u32 hstcp_ssthresh(struct sock *sk)
152{
153	const struct tcp_sock *tp = tcp_sk(sk);
154	struct hstcp *ca = inet_csk_ca(sk);
155
156	/* Do multiplicative decrease */
157	return max(tcp_snd_cwnd(tp) - ((tcp_snd_cwnd(tp) * hstcp_aimd_vals[ca->ai].md) >> 8), 2U);
158}
159
160static struct tcp_congestion_ops tcp_highspeed __read_mostly = {
161	.init		= hstcp_init,
162	.ssthresh	= hstcp_ssthresh,
163	.undo_cwnd	= tcp_reno_undo_cwnd,
164	.cong_avoid	= hstcp_cong_avoid,
165
166	.owner		= THIS_MODULE,
167	.name		= "highspeed"
168};
169
170static int __init hstcp_register(void)
171{
172	BUILD_BUG_ON(sizeof(struct hstcp) > ICSK_CA_PRIV_SIZE);
173	return tcp_register_congestion_control(&tcp_highspeed);
174}
175
176static void __exit hstcp_unregister(void)
177{
178	tcp_unregister_congestion_control(&tcp_highspeed);
179}
180
181module_init(hstcp_register);
182module_exit(hstcp_unregister);
183
184MODULE_AUTHOR("John Heffner");
185MODULE_LICENSE("GPL");
186MODULE_DESCRIPTION("High Speed TCP");
187