1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Binary Increase Congestion control for TCP
4 * Home page:
5 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
6 * This is from the implementation of BICTCP in
7 * Lison-Xu, Kahaled Harfoush, and Injong Rhee.
8 *  "Binary Increase Congestion Control for Fast, Long Distance
9 *  Networks" in InfoComm 2004
10 * Available from:
11 *  http://netsrv.csc.ncsu.edu/export/bitcp.pdf
12 *
13 * Unless BIC is enabled and congestion window is large
14 * this behaves the same as the original Reno.
15 */
16
17#include <linux/mm.h>
18#include <linux/module.h>
19#include <net/tcp.h>
20
21#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
22					 * max_cwnd = snd_cwnd * beta
23					 */
24#define BICTCP_B		4	 /*
25					  * In binary search,
26					  * go to point (max+min)/N
27					  */
28
29static int fast_convergence = 1;
30static int max_increment = 16;
31static int low_window = 14;
32static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
33static int initial_ssthresh;
34static int smooth_part = 20;
35
36module_param(fast_convergence, int, 0644);
37MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
38module_param(max_increment, int, 0644);
39MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
40module_param(low_window, int, 0644);
41MODULE_PARM_DESC(low_window, "lower bound on congestion window (for TCP friendliness)");
42module_param(beta, int, 0644);
43MODULE_PARM_DESC(beta, "beta for multiplicative increase");
44module_param(initial_ssthresh, int, 0644);
45MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
46module_param(smooth_part, int, 0644);
47MODULE_PARM_DESC(smooth_part, "log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax");
48
49/* BIC TCP Parameters */
50struct bictcp {
51	u32	cnt;		/* increase cwnd by 1 after ACKs */
52	u32	last_max_cwnd;	/* last maximum snd_cwnd */
53	u32	last_cwnd;	/* the last snd_cwnd */
54	u32	last_time;	/* time when updated last_cwnd */
55	u32	epoch_start;	/* beginning of an epoch */
56#define ACK_RATIO_SHIFT	4
57	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
58};
59
60static inline void bictcp_reset(struct bictcp *ca)
61{
62	ca->cnt = 0;
63	ca->last_max_cwnd = 0;
64	ca->last_cwnd = 0;
65	ca->last_time = 0;
66	ca->epoch_start = 0;
67	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
68}
69
70static void bictcp_init(struct sock *sk)
71{
72	struct bictcp *ca = inet_csk_ca(sk);
73
74	bictcp_reset(ca);
75
76	if (initial_ssthresh)
77		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
78}
79
80/*
81 * Compute congestion window to use.
82 */
83static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
84{
85	if (ca->last_cwnd == cwnd &&
86	    (s32)(tcp_jiffies32 - ca->last_time) <= HZ / 32)
87		return;
88
89	ca->last_cwnd = cwnd;
90	ca->last_time = tcp_jiffies32;
91
92	if (ca->epoch_start == 0) /* record the beginning of an epoch */
93		ca->epoch_start = tcp_jiffies32;
94
95	/* start off normal */
96	if (cwnd <= low_window) {
97		ca->cnt = cwnd;
98		return;
99	}
100
101	/* binary increase */
102	if (cwnd < ca->last_max_cwnd) {
103		__u32	dist = (ca->last_max_cwnd - cwnd)
104			/ BICTCP_B;
105
106		if (dist > max_increment)
107			/* linear increase */
108			ca->cnt = cwnd / max_increment;
109		else if (dist <= 1U)
110			/* binary search increase */
111			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
112		else
113			/* binary search increase */
114			ca->cnt = cwnd / dist;
115	} else {
116		/* slow start AMD linear increase */
117		if (cwnd < ca->last_max_cwnd + BICTCP_B)
118			/* slow start */
119			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
120		else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
121			/* slow start */
122			ca->cnt = (cwnd * (BICTCP_B-1))
123				/ (cwnd - ca->last_max_cwnd);
124		else
125			/* linear increase */
126			ca->cnt = cwnd / max_increment;
127	}
128
129	/* if in slow start or link utilization is very low */
130	if (ca->last_max_cwnd == 0) {
131		if (ca->cnt > 20) /* increase cwnd 5% per RTT */
132			ca->cnt = 20;
133	}
134
135	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
136	if (ca->cnt == 0)			/* cannot be zero */
137		ca->cnt = 1;
138}
139
140static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
141{
142	struct tcp_sock *tp = tcp_sk(sk);
143	struct bictcp *ca = inet_csk_ca(sk);
144
145	if (!tcp_is_cwnd_limited(sk))
146		return;
147
148	if (tcp_in_slow_start(tp)) {
149		acked = tcp_slow_start(tp, acked);
150		if (!acked)
151			return;
152	}
153	bictcp_update(ca, tcp_snd_cwnd(tp));
154	tcp_cong_avoid_ai(tp, ca->cnt, acked);
155}
156
157/*
158 *	behave like Reno until low_window is reached,
159 *	then increase congestion window slowly
160 */
161static u32 bictcp_recalc_ssthresh(struct sock *sk)
162{
163	const struct tcp_sock *tp = tcp_sk(sk);
164	struct bictcp *ca = inet_csk_ca(sk);
165
166	ca->epoch_start = 0;	/* end of epoch */
167
168	/* Wmax and fast convergence */
169	if (tcp_snd_cwnd(tp) < ca->last_max_cwnd && fast_convergence)
170		ca->last_max_cwnd = (tcp_snd_cwnd(tp) * (BICTCP_BETA_SCALE + beta))
171			/ (2 * BICTCP_BETA_SCALE);
172	else
173		ca->last_max_cwnd = tcp_snd_cwnd(tp);
174
175	if (tcp_snd_cwnd(tp) <= low_window)
176		return max(tcp_snd_cwnd(tp) >> 1U, 2U);
177	else
178		return max((tcp_snd_cwnd(tp) * beta) / BICTCP_BETA_SCALE, 2U);
179}
180
181static void bictcp_state(struct sock *sk, u8 new_state)
182{
183	if (new_state == TCP_CA_Loss)
184		bictcp_reset(inet_csk_ca(sk));
185}
186
187/* Track delayed acknowledgment ratio using sliding window
188 * ratio = (15*ratio + sample) / 16
189 */
190static void bictcp_acked(struct sock *sk, const struct ack_sample *sample)
191{
192	const struct inet_connection_sock *icsk = inet_csk(sk);
193
194	if (icsk->icsk_ca_state == TCP_CA_Open) {
195		struct bictcp *ca = inet_csk_ca(sk);
196
197		ca->delayed_ack += sample->pkts_acked -
198			(ca->delayed_ack >> ACK_RATIO_SHIFT);
199	}
200}
201
202static struct tcp_congestion_ops bictcp __read_mostly = {
203	.init		= bictcp_init,
204	.ssthresh	= bictcp_recalc_ssthresh,
205	.cong_avoid	= bictcp_cong_avoid,
206	.set_state	= bictcp_state,
207	.undo_cwnd	= tcp_reno_undo_cwnd,
208	.pkts_acked     = bictcp_acked,
209	.owner		= THIS_MODULE,
210	.name		= "bic",
211};
212
213static int __init bictcp_register(void)
214{
215	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
216	return tcp_register_congestion_control(&bictcp);
217}
218
219static void __exit bictcp_unregister(void)
220{
221	tcp_unregister_congestion_control(&bictcp);
222}
223
224module_init(bictcp_register);
225module_exit(bictcp_unregister);
226
227MODULE_AUTHOR("Stephen Hemminger");
228MODULE_LICENSE("GPL");
229MODULE_DESCRIPTION("BIC TCP");
230