xref: /openbmc/linux/net/ipv4/tcp_bic.c (revision 7fc96d71)
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  
29  static int fast_convergence = 1;
30  static int max_increment = 16;
31  static int low_window = 14;
32  static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
33  static int initial_ssthresh;
34  static int smooth_part = 20;
35  
36  module_param(fast_convergence, int, 0644);
37  MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
38  module_param(max_increment, int, 0644);
39  MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
40  module_param(low_window, int, 0644);
41  MODULE_PARM_DESC(low_window, "lower bound on congestion window (for TCP friendliness)");
42  module_param(beta, int, 0644);
43  MODULE_PARM_DESC(beta, "beta for multiplicative increase");
44  module_param(initial_ssthresh, int, 0644);
45  MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
46  module_param(smooth_part, int, 0644);
47  MODULE_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 */
50  struct 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  
60  static 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  
70  static 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   */
83  static 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  
140  static 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   */
161  static 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  
181  static 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   */
190  static 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  
202  static 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  
213  static 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  
219  static void __exit bictcp_unregister(void)
220  {
221  	tcp_unregister_congestion_control(&bictcp);
222  }
223  
224  module_init(bictcp_register);
225  module_exit(bictcp_unregister);
226  
227  MODULE_AUTHOR("Stephen Hemminger");
228  MODULE_LICENSE("GPL");
229  MODULE_DESCRIPTION("BIC TCP");
230