1/* DataCenter TCP (DCTCP) congestion control.
2 *
3 * http://simula.stanford.edu/~alizade/Site/DCTCP.html
4 *
5 * This is an implementation of DCTCP over Reno, an enhancement to the
6 * TCP congestion control algorithm designed for data centers. DCTCP
7 * leverages Explicit Congestion Notification (ECN) in the network to
8 * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
9 * the following three data center transport requirements:
10 *
11 *  - High burst tolerance (incast due to partition/aggregate)
12 *  - Low latency (short flows, queries)
13 *  - High throughput (continuous data updates, large file transfers)
14 *    with commodity shallow buffered switches
15 *
16 * The algorithm is described in detail in the following two papers:
17 *
18 * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
19 *    Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
20 *      "Data Center TCP (DCTCP)", Data Center Networks session
21 *      Proc. ACM SIGCOMM, New Delhi, 2010.
22 *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
23 *
24 * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
25 *      "Analysis of DCTCP: Stability, Convergence, and Fairness"
26 *      Proc. ACM SIGMETRICS, San Jose, 2011.
27 *   http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
28 *
29 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
30 *
31 * Authors:
32 *
33 *	Daniel Borkmann <dborkman@redhat.com>
34 *	Florian Westphal <fw@strlen.de>
35 *	Glenn Judd <glenn.judd@morganstanley.com>
36 *
37 * This program is free software; you can redistribute it and/or modify
38 * it under the terms of the GNU General Public License as published by
39 * the Free Software Foundation; either version 2 of the License, or (at
40 * your option) any later version.
41 */
42
43#include <linux/module.h>
44#include <linux/mm.h>
45#include <net/tcp.h>
46#include <linux/inet_diag.h>
47
48#define DCTCP_MAX_ALPHA	1024U
49
50struct dctcp {
51	u32 acked_bytes_ecn;
52	u32 acked_bytes_total;
53	u32 prior_snd_una;
54	u32 prior_rcv_nxt;
55	u32 dctcp_alpha;
56	u32 next_seq;
57	u32 ce_state;
58	u32 delayed_ack_reserved;
59};
60
61static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
62module_param(dctcp_shift_g, uint, 0644);
63MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
64
65static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
66module_param(dctcp_alpha_on_init, uint, 0644);
67MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
68
69static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
70module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
71MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
72		 "parameter for clamping alpha on loss");
73
74static struct tcp_congestion_ops dctcp_reno;
75
76static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
77{
78	ca->next_seq = tp->snd_nxt;
79
80	ca->acked_bytes_ecn = 0;
81	ca->acked_bytes_total = 0;
82}
83
84static void dctcp_init(struct sock *sk)
85{
86	const struct tcp_sock *tp = tcp_sk(sk);
87
88	if ((tp->ecn_flags & TCP_ECN_OK) ||
89	    (sk->sk_state == TCP_LISTEN ||
90	     sk->sk_state == TCP_CLOSE)) {
91		struct dctcp *ca = inet_csk_ca(sk);
92
93		ca->prior_snd_una = tp->snd_una;
94		ca->prior_rcv_nxt = tp->rcv_nxt;
95
96		ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
97
98		ca->delayed_ack_reserved = 0;
99		ca->ce_state = 0;
100
101		dctcp_reset(tp, ca);
102		return;
103	}
104
105	/* No ECN support? Fall back to Reno. Also need to clear
106	 * ECT from sk since it is set during 3WHS for DCTCP.
107	 */
108	inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
109	INET_ECN_dontxmit(sk);
110}
111
112static u32 dctcp_ssthresh(struct sock *sk)
113{
114	const struct dctcp *ca = inet_csk_ca(sk);
115	struct tcp_sock *tp = tcp_sk(sk);
116
117	return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
118}
119
120/* Minimal DCTP CE state machine:
121 *
122 * S:	0 <- last pkt was non-CE
123 *	1 <- last pkt was CE
124 */
125
126static void dctcp_ce_state_0_to_1(struct sock *sk)
127{
128	struct dctcp *ca = inet_csk_ca(sk);
129	struct tcp_sock *tp = tcp_sk(sk);
130
131	/* State has changed from CE=0 to CE=1 and delayed
132	 * ACK has not sent yet.
133	 */
134	if (!ca->ce_state && ca->delayed_ack_reserved) {
135		u32 tmp_rcv_nxt;
136
137		/* Save current rcv_nxt. */
138		tmp_rcv_nxt = tp->rcv_nxt;
139
140		/* Generate previous ack with CE=0. */
141		tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
142		tp->rcv_nxt = ca->prior_rcv_nxt;
143
144		tcp_send_ack(sk);
145
146		/* Recover current rcv_nxt. */
147		tp->rcv_nxt = tmp_rcv_nxt;
148	}
149
150	ca->prior_rcv_nxt = tp->rcv_nxt;
151	ca->ce_state = 1;
152
153	tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
154}
155
156static void dctcp_ce_state_1_to_0(struct sock *sk)
157{
158	struct dctcp *ca = inet_csk_ca(sk);
159	struct tcp_sock *tp = tcp_sk(sk);
160
161	/* State has changed from CE=1 to CE=0 and delayed
162	 * ACK has not sent yet.
163	 */
164	if (ca->ce_state && ca->delayed_ack_reserved) {
165		u32 tmp_rcv_nxt;
166
167		/* Save current rcv_nxt. */
168		tmp_rcv_nxt = tp->rcv_nxt;
169
170		/* Generate previous ack with CE=1. */
171		tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
172		tp->rcv_nxt = ca->prior_rcv_nxt;
173
174		tcp_send_ack(sk);
175
176		/* Recover current rcv_nxt. */
177		tp->rcv_nxt = tmp_rcv_nxt;
178	}
179
180	ca->prior_rcv_nxt = tp->rcv_nxt;
181	ca->ce_state = 0;
182
183	tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
184}
185
186static void dctcp_update_alpha(struct sock *sk, u32 flags)
187{
188	const struct tcp_sock *tp = tcp_sk(sk);
189	struct dctcp *ca = inet_csk_ca(sk);
190	u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
191
192	/* If ack did not advance snd_una, count dupack as MSS size.
193	 * If ack did update window, do not count it at all.
194	 */
195	if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
196		acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
197	if (acked_bytes) {
198		ca->acked_bytes_total += acked_bytes;
199		ca->prior_snd_una = tp->snd_una;
200
201		if (flags & CA_ACK_ECE)
202			ca->acked_bytes_ecn += acked_bytes;
203	}
204
205	/* Expired RTT */
206	if (!before(tp->snd_una, ca->next_seq)) {
207		/* For avoiding denominator == 1. */
208		if (ca->acked_bytes_total == 0)
209			ca->acked_bytes_total = 1;
210
211		/* alpha = (1 - g) * alpha + g * F */
212		ca->dctcp_alpha = ca->dctcp_alpha -
213				  (ca->dctcp_alpha >> dctcp_shift_g) +
214				  (ca->acked_bytes_ecn << (10U - dctcp_shift_g)) /
215				  ca->acked_bytes_total;
216
217		if (ca->dctcp_alpha > DCTCP_MAX_ALPHA)
218			/* Clamp dctcp_alpha to max. */
219			ca->dctcp_alpha = DCTCP_MAX_ALPHA;
220
221		dctcp_reset(tp, ca);
222	}
223}
224
225static void dctcp_state(struct sock *sk, u8 new_state)
226{
227	if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
228		struct dctcp *ca = inet_csk_ca(sk);
229
230		/* If this extension is enabled, we clamp dctcp_alpha to
231		 * max on packet loss; the motivation is that dctcp_alpha
232		 * is an indicator to the extend of congestion and packet
233		 * loss is an indicator of extreme congestion; setting
234		 * this in practice turned out to be beneficial, and
235		 * effectively assumes total congestion which reduces the
236		 * window by half.
237		 */
238		ca->dctcp_alpha = DCTCP_MAX_ALPHA;
239	}
240}
241
242static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
243{
244	struct dctcp *ca = inet_csk_ca(sk);
245
246	switch (ev) {
247	case CA_EVENT_DELAYED_ACK:
248		if (!ca->delayed_ack_reserved)
249			ca->delayed_ack_reserved = 1;
250		break;
251	case CA_EVENT_NON_DELAYED_ACK:
252		if (ca->delayed_ack_reserved)
253			ca->delayed_ack_reserved = 0;
254		break;
255	default:
256		/* Don't care for the rest. */
257		break;
258	}
259}
260
261static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
262{
263	switch (ev) {
264	case CA_EVENT_ECN_IS_CE:
265		dctcp_ce_state_0_to_1(sk);
266		break;
267	case CA_EVENT_ECN_NO_CE:
268		dctcp_ce_state_1_to_0(sk);
269		break;
270	case CA_EVENT_DELAYED_ACK:
271	case CA_EVENT_NON_DELAYED_ACK:
272		dctcp_update_ack_reserved(sk, ev);
273		break;
274	default:
275		/* Don't care for the rest. */
276		break;
277	}
278}
279
280static void dctcp_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
281{
282	const struct dctcp *ca = inet_csk_ca(sk);
283
284	/* Fill it also in case of VEGASINFO due to req struct limits.
285	 * We can still correctly retrieve it later.
286	 */
287	if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
288	    ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
289		struct tcp_dctcp_info info;
290
291		memset(&info, 0, sizeof(info));
292		if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
293			info.dctcp_enabled = 1;
294			info.dctcp_ce_state = (u16) ca->ce_state;
295			info.dctcp_alpha = ca->dctcp_alpha;
296			info.dctcp_ab_ecn = ca->acked_bytes_ecn;
297			info.dctcp_ab_tot = ca->acked_bytes_total;
298		}
299
300		nla_put(skb, INET_DIAG_DCTCPINFO, sizeof(info), &info);
301	}
302}
303
304static struct tcp_congestion_ops dctcp __read_mostly = {
305	.init		= dctcp_init,
306	.in_ack_event   = dctcp_update_alpha,
307	.cwnd_event	= dctcp_cwnd_event,
308	.ssthresh	= dctcp_ssthresh,
309	.cong_avoid	= tcp_reno_cong_avoid,
310	.set_state	= dctcp_state,
311	.get_info	= dctcp_get_info,
312	.flags		= TCP_CONG_NEEDS_ECN,
313	.owner		= THIS_MODULE,
314	.name		= "dctcp",
315};
316
317static struct tcp_congestion_ops dctcp_reno __read_mostly = {
318	.ssthresh	= tcp_reno_ssthresh,
319	.cong_avoid	= tcp_reno_cong_avoid,
320	.get_info	= dctcp_get_info,
321	.owner		= THIS_MODULE,
322	.name		= "dctcp-reno",
323};
324
325static int __init dctcp_register(void)
326{
327	BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
328	return tcp_register_congestion_control(&dctcp);
329}
330
331static void __exit dctcp_unregister(void)
332{
333	tcp_unregister_congestion_control(&dctcp);
334}
335
336module_init(dctcp_register);
337module_exit(dctcp_unregister);
338
339MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
340MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
341MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
342
343MODULE_LICENSE("GPL v2");
344MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");
345