tcp_bic.c revision 35e8694198ba94b62df8aa35fa6e52a1cfb86df2 
1/*
2 * Binary Increase Congestion control for TCP
3 *
4 * This is from the implementation of BICTCP in
5 * Lison-Xu, Kahaled Harfoush, and Injong Rhee.
6 *  "Binary Increase Congestion Control for Fast, Long Distance
7 *  Networks" in InfoComm 2004
8 * Available from:
9 *  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp.pdf
10 *
11 * Unless BIC is enabled and congestion window is large
12 * this behaves the same as the original Reno.
13 */
14
15#include <linux/mm.h>
16#include <linux/module.h>
17#include <net/tcp.h>
18
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
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	loss_cwnd;	/* congestion window at last loss */
54	u32	last_cwnd;	/* the last snd_cwnd */
55	u32	last_time;	/* time when updated last_cwnd */
56	u32	epoch_start;	/* beginning of an epoch */
57#define ACK_RATIO_SHIFT	4
58	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
59};
60
61static inline void bictcp_reset(struct bictcp *ca)
62{
63	ca->cnt = 0;
64	ca->last_max_cwnd = 0;
65	ca->loss_cwnd = 0;
66	ca->last_cwnd = 0;
67	ca->last_time = 0;
68	ca->epoch_start = 0;
69	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
70}
71
72static void bictcp_init(struct sock *sk)
73{
74	bictcp_reset(inet_csk_ca(sk));
75	if (initial_ssthresh)
76		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
77}
78
79/*
80 * Compute congestion window to use.
81 */
82static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
83{
84	if (ca->last_cwnd == cwnd &&
85	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
86		return;
87
88	ca->last_cwnd = cwnd;
89	ca->last_time = tcp_time_stamp;
90
91	if (ca->epoch_start == 0) /* record the beginning of an epoch */
92		ca->epoch_start = tcp_time_stamp;
93
94	/* start off normal */
95	if (cwnd <= low_window) {
96		ca->cnt = cwnd;
97		return;
98	}
99
100	/* binary increase */
101	if (cwnd < ca->last_max_cwnd) {
102		__u32 	dist = (ca->last_max_cwnd - cwnd)
103			/ BICTCP_B;
104
105		if (dist > max_increment)
106			/* linear increase */
107			ca->cnt = cwnd / max_increment;
108		else if (dist <= 1U)
109			/* binary search increase */
110			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
111		else
112			/* binary search increase */
113			ca->cnt = cwnd / dist;
114	} else {
115		/* slow start AMD linear increase */
116		if (cwnd < ca->last_max_cwnd + BICTCP_B)
117			/* slow start */
118			ca->cnt = (cwnd * smooth_part) / BICTCP_B;
119		else if (cwnd < ca->last_max_cwnd + max_increment*(BICTCP_B-1))
120			/* slow start */
121			ca->cnt = (cwnd * (BICTCP_B-1))
122				/ (cwnd - ca->last_max_cwnd);
123		else
124			/* linear increase */
125			ca->cnt = cwnd / max_increment;
126	}
127
128	/* if in slow start or link utilization is very low */
129	if (ca->loss_cwnd == 0) {
130		if (ca->cnt > 20) /* increase cwnd 5% per RTT */
131			ca->cnt = 20;
132	}
133
134	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
135	if (ca->cnt == 0)			/* cannot be zero */
136		ca->cnt = 1;
137}
138
139static void bictcp_cong_avoid(struct sock *sk, u32 ack,
140			      u32 in_flight, int data_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, in_flight))
146		return;
147
148	if (tp->snd_cwnd <= tp->snd_ssthresh)
149		tcp_slow_start(tp);
150	else {
151		bictcp_update(ca, tp->snd_cwnd);
152
153		/* In dangerous area, increase slowly.
154		 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
155		 */
156		if (tp->snd_cwnd_cnt >= ca->cnt) {
157			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
158				tp->snd_cwnd++;
159			tp->snd_cwnd_cnt = 0;
160		} else
161			tp->snd_cwnd_cnt++;
162	}
163
164}
165
166/*
167 *	behave like Reno until low_window is reached,
168 *	then increase congestion window slowly
169 */
170static u32 bictcp_recalc_ssthresh(struct sock *sk)
171{
172	const struct tcp_sock *tp = tcp_sk(sk);
173	struct bictcp *ca = inet_csk_ca(sk);
174
175	ca->epoch_start = 0;	/* end of epoch */
176
177	/* Wmax and fast convergence */
178	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
179		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
180			/ (2 * BICTCP_BETA_SCALE);
181	else
182		ca->last_max_cwnd = tp->snd_cwnd;
183
184	ca->loss_cwnd = tp->snd_cwnd;
185
186
187	if (tp->snd_cwnd <= low_window)
188		return max(tp->snd_cwnd >> 1U, 2U);
189	else
190		return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
191}
192
193static u32 bictcp_undo_cwnd(struct sock *sk)
194{
195	const struct tcp_sock *tp = tcp_sk(sk);
196	const struct bictcp *ca = inet_csk_ca(sk);
197	return max(tp->snd_cwnd, ca->last_max_cwnd);
198}
199
200static void bictcp_state(struct sock *sk, u8 new_state)
201{
202	if (new_state == TCP_CA_Loss)
203		bictcp_reset(inet_csk_ca(sk));
204}
205
206/* Track delayed acknowledgment ratio using sliding window
207 * ratio = (15*ratio + sample) / 16
208 */
209static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt)
210{
211	const struct inet_connection_sock *icsk = inet_csk(sk);
212
213	if (icsk->icsk_ca_state == TCP_CA_Open) {
214		struct bictcp *ca = inet_csk_ca(sk);
215		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
216		ca->delayed_ack += cnt;
217	}
218}
219
220
221static struct tcp_congestion_ops bictcp = {
222	.init		= bictcp_init,
223	.ssthresh	= bictcp_recalc_ssthresh,
224	.cong_avoid	= bictcp_cong_avoid,
225	.set_state	= bictcp_state,
226	.undo_cwnd	= bictcp_undo_cwnd,
227	.pkts_acked     = bictcp_acked,
228	.owner		= THIS_MODULE,
229	.name		= "bic",
230};
231
232static int __init bictcp_register(void)
233{
234	BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
235	return tcp_register_congestion_control(&bictcp);
236}
237
238static void __exit bictcp_unregister(void)
239{
240	tcp_unregister_congestion_control(&bictcp);
241}
242
243module_init(bictcp_register);
244module_exit(bictcp_unregister);
245
246MODULE_AUTHOR("Stephen Hemminger");
247MODULE_LICENSE("GPL");
248MODULE_DESCRIPTION("BIC TCP");
249