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1// SPDX-License-Identifier: GPL-2.0-or-later
2/* DataCenter TCP (DCTCP) congestion control.
3 *
4 * http://simula.stanford.edu/~alizade/Site/DCTCP.html
5 *
6 * This is an implementation of DCTCP over Reno, an enhancement to the
7 * TCP congestion control algorithm designed for data centers. DCTCP
8 * leverages Explicit Congestion Notification (ECN) in the network to
9 * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
10 * the following three data center transport requirements:
11 *
12 * - High burst tolerance (incast due to partition/aggregate)
13 * - Low latency (short flows, queries)
14 * - High throughput (continuous data updates, large file transfers)
15 * with commodity shallow buffered switches
16 *
17 * The algorithm is described in detail in the following two papers:
18 *
19 * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
20 * Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
21 * "Data Center TCP (DCTCP)", Data Center Networks session
22 * Proc. ACM SIGCOMM, New Delhi, 2010.
23 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
24 *
25 * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
26 * "Analysis of DCTCP: Stability, Convergence, and Fairness"
27 * Proc. ACM SIGMETRICS, San Jose, 2011.
28 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
29 *
30 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
31 *
32 * Authors:
33 *
34 * Daniel Borkmann <dborkman@redhat.com>
35 * Florian Westphal <fw@strlen.de>
36 * Glenn Judd <glenn.judd@morganstanley.com>
37 */
38
39#include <linux/module.h>
40#include <linux/mm.h>
41#include <net/tcp.h>
42#include <linux/inet_diag.h>
43#include "tcp_dctcp.h"
44
45#define DCTCP_MAX_ALPHA 1024U
46
47struct dctcp {
48 u32 old_delivered;
49 u32 old_delivered_ce;
50 u32 prior_rcv_nxt;
51 u32 dctcp_alpha;
52 u32 next_seq;
53 u32 ce_state;
54 u32 loss_cwnd;
55};
56
57static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
58module_param(dctcp_shift_g, uint, 0644);
59MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
60
61static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
62module_param(dctcp_alpha_on_init, uint, 0644);
63MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
64
65static struct tcp_congestion_ops dctcp_reno;
66
67static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
68{
69 ca->next_seq = tp->snd_nxt;
70
71 ca->old_delivered = tp->delivered;
72 ca->old_delivered_ce = tp->delivered_ce;
73}
74
75static void dctcp_init(struct sock *sk)
76{
77 const struct tcp_sock *tp = tcp_sk(sk);
78
79 if ((tp->ecn_flags & TCP_ECN_OK) ||
80 (sk->sk_state == TCP_LISTEN ||
81 sk->sk_state == TCP_CLOSE)) {
82 struct dctcp *ca = inet_csk_ca(sk);
83
84 ca->prior_rcv_nxt = tp->rcv_nxt;
85
86 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
87
88 ca->loss_cwnd = 0;
89 ca->ce_state = 0;
90
91 dctcp_reset(tp, ca);
92 return;
93 }
94
95 /* No ECN support? Fall back to Reno. Also need to clear
96 * ECT from sk since it is set during 3WHS for DCTCP.
97 */
98 inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
99 INET_ECN_dontxmit(sk);
100}
101
102static u32 dctcp_ssthresh(struct sock *sk)
103{
104 struct dctcp *ca = inet_csk_ca(sk);
105 struct tcp_sock *tp = tcp_sk(sk);
106
107 ca->loss_cwnd = tp->snd_cwnd;
108 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
109}
110
111static void dctcp_update_alpha(struct sock *sk, u32 flags)
112{
113 const struct tcp_sock *tp = tcp_sk(sk);
114 struct dctcp *ca = inet_csk_ca(sk);
115
116 /* Expired RTT */
117 if (!before(tp->snd_una, ca->next_seq)) {
118 u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce;
119 u32 alpha = ca->dctcp_alpha;
120
121 /* alpha = (1 - g) * alpha + g * F */
122
123 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
124 if (delivered_ce) {
125 u32 delivered = tp->delivered - ca->old_delivered;
126
127 /* If dctcp_shift_g == 1, a 32bit value would overflow
128 * after 8 M packets.
129 */
130 delivered_ce <<= (10 - dctcp_shift_g);
131 delivered_ce /= max(1U, delivered);
132
133 alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA);
134 }
135 /* dctcp_alpha can be read from dctcp_get_info() without
136 * synchro, so we ask compiler to not use dctcp_alpha
137 * as a temporary variable in prior operations.
138 */
139 WRITE_ONCE(ca->dctcp_alpha, alpha);
140 dctcp_reset(tp, ca);
141 }
142}
143
144static void dctcp_react_to_loss(struct sock *sk)
145{
146 struct dctcp *ca = inet_csk_ca(sk);
147 struct tcp_sock *tp = tcp_sk(sk);
148
149 ca->loss_cwnd = tp->snd_cwnd;
150 tp->snd_ssthresh = max(tp->snd_cwnd >> 1U, 2U);
151}
152
153static void dctcp_state(struct sock *sk, u8 new_state)
154{
155 if (new_state == TCP_CA_Recovery &&
156 new_state != inet_csk(sk)->icsk_ca_state)
157 dctcp_react_to_loss(sk);
158 /* We handle RTO in dctcp_cwnd_event to ensure that we perform only
159 * one loss-adjustment per RTT.
160 */
161}
162
163static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
164{
165 struct dctcp *ca = inet_csk_ca(sk);
166
167 switch (ev) {
168 case CA_EVENT_ECN_IS_CE:
169 case CA_EVENT_ECN_NO_CE:
170 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
171 break;
172 case CA_EVENT_LOSS:
173 dctcp_react_to_loss(sk);
174 break;
175 default:
176 /* Don't care for the rest. */
177 break;
178 }
179}
180
181static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
182 union tcp_cc_info *info)
183{
184 const struct dctcp *ca = inet_csk_ca(sk);
185 const struct tcp_sock *tp = tcp_sk(sk);
186
187 /* Fill it also in case of VEGASINFO due to req struct limits.
188 * We can still correctly retrieve it later.
189 */
190 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
191 ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
192 memset(&info->dctcp, 0, sizeof(info->dctcp));
193 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
194 info->dctcp.dctcp_enabled = 1;
195 info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
196 info->dctcp.dctcp_alpha = ca->dctcp_alpha;
197 info->dctcp.dctcp_ab_ecn = tp->mss_cache *
198 (tp->delivered_ce - ca->old_delivered_ce);
199 info->dctcp.dctcp_ab_tot = tp->mss_cache *
200 (tp->delivered - ca->old_delivered);
201 }
202
203 *attr = INET_DIAG_DCTCPINFO;
204 return sizeof(info->dctcp);
205 }
206 return 0;
207}
208
209static u32 dctcp_cwnd_undo(struct sock *sk)
210{
211 const struct dctcp *ca = inet_csk_ca(sk);
212
213 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
214}
215
216static struct tcp_congestion_ops dctcp __read_mostly = {
217 .init = dctcp_init,
218 .in_ack_event = dctcp_update_alpha,
219 .cwnd_event = dctcp_cwnd_event,
220 .ssthresh = dctcp_ssthresh,
221 .cong_avoid = tcp_reno_cong_avoid,
222 .undo_cwnd = dctcp_cwnd_undo,
223 .set_state = dctcp_state,
224 .get_info = dctcp_get_info,
225 .flags = TCP_CONG_NEEDS_ECN,
226 .owner = THIS_MODULE,
227 .name = "dctcp",
228};
229
230static struct tcp_congestion_ops dctcp_reno __read_mostly = {
231 .ssthresh = tcp_reno_ssthresh,
232 .cong_avoid = tcp_reno_cong_avoid,
233 .undo_cwnd = tcp_reno_undo_cwnd,
234 .get_info = dctcp_get_info,
235 .owner = THIS_MODULE,
236 .name = "dctcp-reno",
237};
238
239static int __init dctcp_register(void)
240{
241 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
242 return tcp_register_congestion_control(&dctcp);
243}
244
245static void __exit dctcp_unregister(void)
246{
247 tcp_unregister_congestion_control(&dctcp);
248}
249
250module_init(dctcp_register);
251module_exit(dctcp_unregister);
252
253MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
254MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
255MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
256
257MODULE_LICENSE("GPL v2");
258MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");