1/*
2 * Binary Increase Congestion control for TCP
3 * Home page:
4 *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
5 * This is from the implementation of BICTCP in
6 * Lison-Xu, Kahaled Harfoush, and Injong Rhee.
7 *  "Binary Increase Congestion Control for Fast, Long Distance
8 *  Networks" in InfoComm 2004
9 * Available from:
10 *  http://netsrv.csc.ncsu.edu/export/bitcp.pdf
11 *
12 * Unless BIC is enabled and congestion window is large
13 * this behaves the same as the original Reno.
14 */
15
16#include <linux/mm.h>
17#include <linux/module.h>
18#include <net/tcp.h>
19
20#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
21					 * max_cwnd = snd_cwnd * beta
22					 */
23#define BICTCP_B		4	 /*
24					  * In binary search,
25					  * go to point (max+min)/N
26					  */
27
28static int fast_convergence = 1;
29static int max_increment = 16;
30static int low_window = 14;
31static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
32static int initial_ssthresh;
33static int smooth_part = 20;
34
35module_param(fast_convergence, int, 0644);
36MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
37module_param(max_increment, int, 0644);
38MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
39module_param(low_window, int, 0644);
40MODULE_PARM_DESC(low_window, "lower bound on congestion window (for TCP friendliness)");
41module_param(beta, int, 0644);
42MODULE_PARM_DESC(beta, "beta for multiplicative increase");
43module_param(initial_ssthresh, int, 0644);
44MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
45module_param(smooth_part, int, 0644);
46MODULE_PARM_DESC(smooth_part, "log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax");
47
48/* BIC TCP Parameters */
49struct bictcp {
50	u32	cnt;		/* increase cwnd by 1 after ACKs */
51	u32	last_max_cwnd;	/* last maximum snd_cwnd */
52	u32	loss_cwnd;	/* congestion window at last loss */
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	ca->loss_cwnd = 0;
76
77	if (initial_ssthresh)
78		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
79}
80
81/*
82 * Compute congestion window to use.
83 */
84static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
85{
86	if (ca->last_cwnd == cwnd &&
87	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
88		return;
89
90	ca->last_cwnd = cwnd;
91	ca->last_time = tcp_time_stamp;
92
93	if (ca->epoch_start == 0) /* record the beginning of an epoch */
94		ca->epoch_start = tcp_time_stamp;
95
96	/* start off normal */
97	if (cwnd <= low_window) {
98		ca->cnt = cwnd;
99		return;
100	}
101
102	/* binary increase */
103	if (cwnd < ca->last_max_cwnd) {
104		__u32	dist = (ca->last_max_cwnd - cwnd)
105			/ BICTCP_B;
106
107		if (dist > max_increment)
108			/* linear increase */
109			ca->cnt = cwnd / max_increment;
110		else if (dist <= 1U)
111			/* binary search increase */
112			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
113		else
114			/* binary search increase */
115			ca->cnt = cwnd / dist;
116	} else {
117		/* slow start AMD linear increase */
118		if (cwnd < ca->last_max_cwnd + BICTCP_B)
119			/* slow start */
120			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
121		else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
122			/* slow start */
123			ca->cnt = (cwnd * (BICTCP_B-1))
124				/ (cwnd - ca->last_max_cwnd);
125		else
126			/* linear increase */
127			ca->cnt = cwnd / max_increment;
128	}
129
130	/* if in slow start or link utilization is very low */
131	if (ca->last_max_cwnd == 0) {
132		if (ca->cnt > 20) /* increase cwnd 5% per RTT */
133			ca->cnt = 20;
134	}
135
136	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
137	if (ca->cnt == 0)			/* cannot be zero */
138		ca->cnt = 1;
139}
140
141static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
142{
143	struct tcp_sock *tp = tcp_sk(sk);
144	struct bictcp *ca = inet_csk_ca(sk);
145
146	if (!tcp_is_cwnd_limited(sk))
147		return;
148
149	if (tp->snd_cwnd <= tp->snd_ssthresh)
150		tcp_slow_start(tp, acked);
151	else {
152		bictcp_update(ca, tp->snd_cwnd);
153		tcp_cong_avoid_ai(tp, ca->cnt, 1);
154	}
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 (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
170		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
171			/ (2 * BICTCP_BETA_SCALE);
172	else
173		ca->last_max_cwnd = tp->snd_cwnd;
174
175	ca->loss_cwnd = tp->snd_cwnd;
176
177	if (tp->snd_cwnd <= low_window)
178		return max(tp->snd_cwnd >> 1U, 2U);
179	else
180		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
181}
182
183static u32 bictcp_undo_cwnd(struct sock *sk)
184{
185	const struct tcp_sock *tp = tcp_sk(sk);
186	const struct bictcp *ca = inet_csk_ca(sk);
187
188	return max(tp->snd_cwnd, ca->loss_cwnd);
189}
190
191static void bictcp_state(struct sock *sk, u8 new_state)
192{
193	if (new_state == TCP_CA_Loss)
194		bictcp_reset(inet_csk_ca(sk));
195}
196
197/* Track delayed acknowledgment ratio using sliding window
198 * ratio = (15*ratio + sample) / 16
199 */
200static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt)
201{
202	const struct inet_connection_sock *icsk = inet_csk(sk);
203
204	if (icsk->icsk_ca_state == TCP_CA_Open) {
205		struct bictcp *ca = inet_csk_ca(sk);
206
207		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
208		ca->delayed_ack += cnt;
209	}
210}
211
212static struct tcp_congestion_ops bictcp __read_mostly = {
213	.init		= bictcp_init,
214	.ssthresh	= bictcp_recalc_ssthresh,
215	.cong_avoid	= bictcp_cong_avoid,
216	.set_state	= bictcp_state,
217	.undo_cwnd	= bictcp_undo_cwnd,
218	.pkts_acked     = bictcp_acked,
219	.owner		= THIS_MODULE,
220	.name		= "bic",
221};
222
223static int __init bictcp_register(void)
224{
225	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
226	return tcp_register_congestion_control(&bictcp);
227}
228
229static void __exit bictcp_unregister(void)
230{
231	tcp_unregister_congestion_control(&bictcp);
232}
233
234module_init(bictcp_register);
235module_exit(bictcp_unregister);
236
237MODULE_AUTHOR("Stephen Hemminger");
238MODULE_LICENSE("GPL");
239MODULE_DESCRIPTION("BIC TCP");
240